It would require a climb check (DC 15) to stay on or a balance check (DC10) to attack, followed by a grapple check if you wanted to grapple. For the climb check or grapple check it would be necessary to have free limbs. Unless you have a climb speed, you could not attack if you are using climb, but I would allow a grapple attempt. Of course, I am making this up as I go, so if anyone has any other ideas, let me know. If not, my ruling stands.
After reading Infiniti's post, I think you would have to teleport above the dragon and catch yourself on it. Everything else is as I have written. Note that a non-quickened teleport means no grapple or attack attempts.
Round 215
Chuckles stays put.
Chuckles does not dodge the acid (18 Ref save) and takes 27 points of acid damage. Chuckles has 57 HP remaining.
It is ZZ1's turn.
Evilhalfling[sblock]Breath weapon is available next round.[/sblock]