Specific rules for climbing monsters are unnecessary. You can create them by using existing modifiers for regular climbing. Here is some make-believe math.
Climb DC: X
Moving Target: DC +2
Surface difficulty(smooth, slimy, wet, ie: difficult terrain climbing surface): DC +2
Aggressive(ie: fighting against your attempt to climb it, you may need to have the monster make a spot chec, which may increase the DC further if it notices you and acts against you, or simply rule that if you are in combat with it, this applies): +2
So lets say you want to climb atop a dragon you are battling, I'd say dragons are a difficult climb regardless, so lets say that's a DC 18. Dragon scales don't make for the best handholds, so there's a +2. The dragon is likely moving as it is fighting you, so there's another +2, it's also fighting you, so +2.
So climbing a dragon while you are in combat with it, would give you a DC 24. If it succeeds in it's spot check(it gets one every time you move while on it), the DC goes up by another +2.
I don't really think there are any special rules needed for this. I think the outline I wrote above presents a sound way to approach adjusting the DC for attempting to climb a monster.