My player's have (foolishly) attempted to climb onto creatures bigger than themselves so many times. I eventually got fed up and made rules for such a thing. Maybe they'll be of use at your table? Take a look!
I'll keep the most up to date version of these rule on my blog, located here.
There are two categories of creatures referred to within these rules: the summit and the climber. The summit is the creature that is to be climbed upon. The climber refers to the creature attempting to climb onto another creature.
To begin climbing onto a dragon, giant, or other larger creature, a climber must first succeed on a melee touch attack. This attack provokes attacks of opportunity. If you are hit by the summit, you take the damage normally and apply half that amount as a penalty to the attack roll. On a success, you have successfully began to climb onto the summit and entered its space. More than one climber may climb onto a summit, but only if they are all at least two size categories smaller than the summit. For each size category smaller, an additional number of climbers may attempt to climb on the summit. A huge summit may be climbed upon by two medium climbers, four small climbers, eight tiny climbers, and so on. Each climber beyond the first takes a cumulative –4 penalty to their attack and climb rolls to climb onto the summit.
Once you are climbing on the summit, you must make a climb check for each size category larger than you the summit is. The DC for the climb check is equal to 10+the summit’s CR. This is to climb to a position where you are able to hold on properly and still be able to take other actions. You may instead begin to take other actions at any point before making your successful climb checks, but you incur a –4 penalty to all d20 rolls for each climb check that was ignored. For example, when climbing onto a bullete, a human can make two climb checks to get onto its back, make a single climb check to attack on the bullete’s side at a –4, or forgo the climb checks and began attacking at the bullete’s legs at a –8.
Once in position, there are a variety of actions that can be taken. As a standard action, you make a single melee attack with a light or one-handed weapon at a +4 bonus. This bonus increases by 2 for every size category, beyond the first, you are smaller than the summit. For example, a halfling climbing onto an ogre would receive a +6 bonus to her attack roll. Summits attacked in this way are treated as flat-footed for the purposes of sneak attack. Also, as a standard action, you may cast a spell, but must first succeed on a concentration check (DC 15+spell level).
As a full-round action, you may instead choose to target a specific body part to damage. The effects are the same as that of the gunslinger’s targeting deed. Instead as a full-round action, you may brace yourself onto the summit to aid in staying on (see below). This grants you a +8 bonus to climb checks until the start of your next turn.
A summit that is being climbed upon may try to remove a climber or climbers in a number of ways. As a standard action, the summit may make a single melee attack against a climber. When damaged, the climber must make a climb check to hold on. The DC is equal to 5+the damage taken. Instead, the summit may attempt a combat maneuver against the climber. This combat maneuver does not provoke attacks of opportunity. A successful check knocks the climber off. The climber takes 1d6 points of damage for every size category smaller they are than the summit. Finally, as a full-round action, the summit may make a Strength shake to force all climbers off at once. The climbers make a climb check to hold on with the DC equal to the summit’s Strength check.
Any spells or effects that offer a reflex save that target the summit, also target any climbers. Climbers receive a –4 to this reflex save. If a summit is knocked prone or otherwise falls over, climbers may attempt a reflex save (DC equal to 10+the summit’s CR) to leap to an adjacent square safely or remain on the summit, but avoid damage. Failure results in taking damage as if knocked off the summit (see above).
This seems pretty complicated. One of the things that was changed in pathfinder was that a lot of things were turned into combat maneuvers. When I have had PC's want to do this I just have them make combat maneuver checks. Or maybe they could make a climb check vs the monsters CMD.