[turbo6bar]

Just brainstorming here, but can you get under the stairs? Could you remove the drywall and fasten the treads from beneath? Fabricate brackets from angle iron, predrill, and fasten to the stringers. Then, cut treads to template, prefinish, fasten treads from beneath, and then refinish drywall. A decent drywall guy could rehang and finish drywall faster than you could finish the treads in place. Sucks to be the future guy who has to rip out the treads that are screwed down, though.