Toady's planning it, but it's one of those "eventually" things.
Climbing colossi is an interesting idea, but one that will need to wait for (for instance) a better idea of how wrestling "works," and making creatures' sizes count more (currently a kobold can kick a giant in the head).
As far as scaling creatures goes, all that you need is an abstract version of how you deal with large monsters in Dragons Dogma. The first step is to separate grappling and wrestling. Grappling is a test of strength and wills that consist of a take-down attempt and the associated counter, while wrestling would be what happens on the ground after a successful grapple ie pins, submissions, just intentionally snapping bones and tendons ect. The best example of this is MMA with most grappling being against a cage.
When it actually comes to scaling a giant, the it would run multiple checks: 1. Does the giant simply shrug of the attempt? If yes, then is the would be climber knocked prone? If yes then a
horrible death great fun will likely ensue. 2. Does the attempt succeed? If yes, the climber can spend a step climbing a little higher or launch an attack of an appropriate sort. After 2. the giant gets to attempt
a pest extraction operation counter of some some sort with the appropriate results for success/failure. This would continue until either the climber has been removed or the giant is killed. (CAUTION!!) Falling giants have been known to squash/pin those who climbed them and many a hapless passerby. Dwarf bowling anyone?