The immobilization DC could be the Save DC. I don't remember if that's the current system or not, but it could work. It would max out at 20-plus wis mod. (from experience, that makes about 27, without exotic races or epic levels.)
Assuming a +4 wis mod, an average strength caster(read, 10) would be able to slowly work their way out until bender level 12, roughly. This might not work against big strong critters, though. Stuff with a strength of 30 or more would be able to bust out of confinement in a round or two.
I think matching save progressions against ability checks is a little more sensible than matching it against skill checks. A level 20 character can just barely make a DC 27 ability check, if they're built cleverly. But you need a cleverly built bender to get that DC, so it's all good. If we use the old system, nobody is going to be able to make a DC 40+ ability check, and benders can get 30+ modifiers to bending without even really trying.
On the subject of the new thread, well done Meph. Rules are cleared up, everything is easy to access, and it's a triumph of orderly arrangement, all told. My one beef is that the earthbending color is a rust-brown instead of a dirt brown or green. And we all know how important that is.