my ex was a drywall taper/finisher who had to do some amazing feats with cathedral ceilings. I still "think" like a taper. Since you have a window upstairs, you might be able to make a platform to hang outside the window to stand or sit on, or maybe just lean far enough out that you can aim a sprayer or use a long, long roller on a handle. The platform would be supported by 2x4's inside. Remember those drive-in restaurant trays they would hang over your car window ? ( Sonic, etc)? Something like that. I can make a sketch....
