Okay, I did something like this with my old Kingmaker campaign. Dug up the post from 2011 (!). Here it is.
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This is what I worked up for my KM campaign, which is currently in suspension about halfway through the second module. It makes the following assumptions:
1) The players will be interested in some level of Game-of-Thrones type intrigue back in Brevoy as a supplement to the main campaign; and,
2) Significant amounts of time (at least several years) will pass as the PCs move through the Adventure Path.
I made a number of changes to Brevoy's background and politics. First and foremost I got rid of the "vanishing of the royal family" event. It's an interesting idea, but from a campaign POV it's just a big red herring. Instead, I plugged in a dynastic war a generation or two back, with the current king the son of the victor, and several of the eight noble families still licking their wounds. Next, I set up Brevoy's royal family as follows:
The King -- As in the AP, the King is LN and getting on in years. I made him a widower instead of single. He's a decent enough old stick, but his health is failing and he won't be around much longer. He has three children:
Prince Paul -- ~25 years old, LG, something like an Ari 3/Ftr 3 (not that it matters). Prince Paul is honorable, diligent, intelligent, and good. He'd make a great King -- strong and wise, just yet merciful. Unfortunately, that's not going to happen. For now, though, he should be connected to one or more of the PCs -- he could be the patron of their party. IMC, one of the PCs was a bastard younger cousin of the King; Paul was the only royal willing to acknowledge the connection. In any event, Prince Paul is very interested in the plan to expand into the Stolen Lands.
Duke Mikhail -- ~19 years old, CE. I think he'll start as an Ari 2/Rog 1. (Later on he may pick up enough levels to become an assassin or something.) Duke Mikhail is scrawny, creepy, and mentally unstable. When the campaign begins he'll be more like a bad CN. But he's surrounded by evil companions, and his alignment is going to head steadily south over the course of the campaign. He's going to start out as an irresponsible, obnoxious, spoiled young aristocrat and finish as Caligula.
Princess Margalo -- ~12-13 years old, NG. Ari 1. Later on she'll start manifesting sorcerous abilities, if she survives.
A fourth member of the court bears mentioning: the King's Chancellor, Lord Voris. He's LE and a Wiz 6 (meaning he's just high enough level to register with a Detect Evil spell). He has a pencil-thin mustache and a tendency to treat almost everyone as either a social or intellectual inferior. Voris is a cruel man who never forgets a slight. However, he's also very lawful and very dedicated to his job -- he's ascetic, efficient, honest, and incredibly hard working.
So. At the beginning of play, the PCs should be aware of Prince Paul. They may reasonably assume that he is the sort of patron that low-level characters often have until they're big enough to get iterative attacks and pay for their own Raise Dead spells. Well, yes and no. But mostly no.
The first module will need to have two cutscenes added. Cutscene #1 has the PCs coming to visit the royal court. They don't have to go all the way to the capital for this -- the court can be visiting Restov. (Prince Paul is on excellent terms with the swordlords; not only is he a fighter, and lawful, but he wants to make the town a commercial hub for the whole region by.) It can take place after the PCs have achieved some notable success but before the final confrontation with the Staglord. The PCs will meet all three royal children. Prince Paul will congratulate them, and possibly give them some reward. Duke Mikhail will be a snotty jackhole; he should insult at least one PC and possibly all of them. Play him obnoxious and push it hard, but make sure the players are aware that attacking a royal heir is a Really Bad Idea. A bit later, let them encounter Princess Margalo, who should be played as sympathetic, intelligent, and chafing a bit under the restrictions of her life. Finally, they can run into Lord Voris, who will be cool and a bit contemptuous -- not because he's evil, but because he's not convinced that the whole Stolen Lands thing is a cost-effective use of the kingdom's resources. The PCs may assume that Voris is an Evil Vizier / future BBEG; let them.
Cutscene #2 can take place at any time around the end of the module, either before or just after the confrontation with the Staglord. It consists simply of a messenger arriving, panting and desperate, with terrible news: Prince Paul has disappeared. The disappearance should be sudden and inexplicable; however, once the King starts paying for high-level divination spells, it will be clear that powerful magic was involved.
What's actually happened to the Prince: Prince Paul has been kidnapped by Nyrissa, as part of her plan to take the Stolen Lands. Her reasoning is that removing the royal patron of the plan will nip it in the bud. In a sense, it's the very success of the PCs and the others that has brought this on... Anyway, the Prince is off to the First World, where Nyrissa will keep him as a plaything until she grows bored. Even the highest level magics will be unable to locate him, and he won't be coming back soon if ever.
Allow a year or two to pass. Let the PCs get their new kingdom up and running. The King sends out groups of questing adventurers looking for the missing Prince; have one or two of these pass through the PCs' territory. Don't dwell on it, but keep the missing Prince a subplot in the background.
Cutscene #3, early in the second module: the PCs go to court again, perhaps to receive knighthoods or patents of nobility. They meet the King and find that he is terribly shrunken, obviously fading fast. Duke Mikhail -- or Prince Mikhail, as many are calling him -- is on the rise. Have the PCs meet some of his evil companions. IMC, these included a brutal antipaladin and a grotesquely obese corrupt cleric. (N.B., given PC truculence, encounters with these guys could easily end with one or more PCs doing a bit of dungeon time until Voris can smooth things over.) Princess Margalo is developing minor sorcerous powers; she is keeping this secret, but may confide in a PC. (Margalo is bright and level-headed, but she's still young enough to be impressed by heroic adventurers.) Finally, Lord Voris seems unchanged, still arrogant and condescending... but in fact he's now quietly on the PCs' side; he wants to move the colonization plan forward as fast as possible, as a hedge against the chaos that will ensue when the King dies.
Cutscene #4 should take place around the end of the second module: the King is dead. Long live our good King Mikhail!
Civil war of course ensues, and from the beginning of module 3 onwards the PCs are on their own. That's fine, they're building their kingdom and having adventures and stuff. Bu-ut: sometime around the end of module 3, a desperate pair of riders shows up at the PC's door one stormy knight. One is a loyal knight of the royal whatever, injured and dying. The other is Princess Margalo. The dying knight has a letter from Lord Voris: the increasingly deranged King Mikhail was planning to marry her off to his antipaladin buddy (who is now a Duke and commander of the Royal Army). Voris is sending her to safety before he makes a desperate attempt to remove King Mikhail from power. A few days later comes the news: Lord Voris has been executed, for the horrible crime of killing poor princess Margalo...
The PCs must now keep Margalo safe and concealed while gradually building up a force that can support her claim to the throne... all the while carrying on with the back half of the adventure path, of course. When King Mikhail finds out what's going on, they're going to be invaded toute suite; you can time that for whenever you think the PCs are ready. Note that in Book Six of the AP you can have the PCs rescue Prince Paul (remember him?) from years of weird captivity under Nyrissa. Of course, at this point he conveniently no longer wants to be King. He hands over his signet and goes off to the life of a wandering pilgrim in the First World.
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This requires some work on the DM's part, but my players seemed to enjoy it; YMMV.
cheers,
Doug M.
