Hmm...Well, without a diagram or photo to help me visualize the situation a little more clearly, all I can come up with (barring getting an articulating manlift into the house), would be to set up with ladderjacks and a pick. Werner and a couple other companies make those neat adjustable convertable ladders, "Little Giant" is one of them. I have the Werner model, and it's been a godsend on staircases.
You could also rent some sectional rolling scaffolding. I've done 40 ft. church ceilings with those (there are outriggers to stabilize them if you're using more than a couple of sections).
This doesn't sound like a fun project--I hope that you bid accordingly...