Today I drove over to the top powder coater's shop and he saw the rims for himself. Like the other shop, he was impressed by the factory tinted clear over the machined finish... but sadly, he said he also couldn't do what I was trying to achieve. He said it's difficult to mask because it's a two-piece wheel with small gaps between the barrel and hub. He said my best course of action would be to take it to a rim shop and have them disassemble the rims and send out the barrels to be powder coated. Otherwise, if I chose to do it my way, it would be astronomically expensive.
So, my next course of action is to contact a rim repair shop not to far from home and see if they can disassemble and powder coat the rims. If not, I'm going to leave the rims as-is and focus on getting the body modified to accommodate the rims, and be done with it. And if that's the case, I'll just have everything mounted and balanced and hand them and the vehicle over to the custom fabricator and say "make them fit"... okay just kidding, I'm not packing that kind of swagger or cash.
I'll keep you guys posted as usual.