You've already come up with a solution, but I faced the same problem in my campaign -- a player announced that he wanted to retire his PC and bring in a new one *just* as the party was heading into Kakishon. I was able to plan for it, fortunately, but it took a lot of story wrangling to sort out.
So, for the record: In our campaign, the new PC was (is) born in Kakishon to a group of "adventurers" who were the last mortals to discover and activate the Scroll of Kakishon prior to Jhavhul's exile. The fairly extensive backstory to justify it all (which the player has since basically ignored, sadly) is that his parents went after the last known owner of the Scroll -- a massive blue dragon -- but were overwhelmed and forced to flee into the Scroll. When one or two members of their party attempted to "sneak" back out and never returned, the remainder realized that anyone who left Kakishon was probably popping back into reality literally at the wyrm's feet, so they were effectively trapped. And, soon realizing that they were trapped in paradise, they eventually settled down, giving birth to the PC and raising him to adulthood in the Pleasure Palace before Jhavhul's army suddenly burst in and went on a rampage.
Since I had to explain not just why a replacement PC was kicking around in Kakishon but also 10th level, I told the player his PC had fought a guerrilla war against Jhavhul's army of smoke and flame and the resurgent proteans (while essentially being shunned by Obherak), but when the tide turned against them, the PC was advised to escape death the only way he could: flee to the Serpent Isles, find the Golden Ram, and provoke it into turning him into solid gold. (In my campaign, I house-ruled that anyone hit twice by the Ram's breath weapon is petrified permanently until the Ram is destroyed.)
Cut to roughly 400 years later. The existing PCs enter Kakishon, slay the Golden Ram, and inadvertently restore the new PC to flesh-and-blood.
I also decided that the Clockwork General is (what's left of) the new PC's father. Hoping for a story hook there, but so far the player is actively resisting making anything of it.
Other possible PC ideas that I floated but weren't used:
* An ancient (but minor) enemy of Nex kept in stasis in the Pleasure Palace dungeons, to be released by Dilix to aid the PCs
* An abandoned and resentful descendant of Jhavhul's horde