Cling to the foe and walking on the spine of the dragon

[Pad Shiro]

Why not?

I propose to add to tumble through at page 145 this:
if the enemy is larger than you al least two sizes you can move on them (using also climb if necessary, for example). The foe don't take disadvantages for that and you must make maintain the balance on the reflex DC when necessary.

And i propose to add this to grabble at page 146:
if the enemy il larger than you 3 sizes or more, you can make the attempt, but he do not take the grabbed condition.

[Yolande d'Bar]

I'm not sure about the mechanics the OP mentions, but I'd really like this to be a part of the game.

By which I mean, not gated off behind a feat, but like anyone can attempt to climb along a dragon's back, if it's bigger enough than you. Of course it should be horribly dangerous--but quite epic if you succeed.

One of the very few 5e rules I've wanted to see in PF.

[Pad Shiro]

Climb may be a check against the fortitude DC of the monster.

The difficulty seems to be balanced by the stats of the monsters.