Rusty,
I think it would work, if you coat the interior with poly-modified cement over your cob foundation and frame. Seems like the cob would be a good insulator, but you would have to transition the interior from the silt structure (that water will destroy) to the cement structure and coating. Not sure exactly how that would work, but if the cob frame were sturdy enough, it shouldn't be too much of a problem to do a tough and flexible concrete coating inside the tub. Probably keep it simple on the plumbing, and it should work like any built-in spa. I'm in Boise, too, so if you want to give it a shot, let me know and maybe I can help!
