Made a little progress this week. Using Dave's advice I used compressed air to force the brake pistons out of the calipers. I started with 5 psi and worked upward from there. It got scary before they came apart. In the end I had them face down on the garage floor, on a furniture blanket, with my foot on them. It took 120 psi before they popped out! The pistons have a little discoloration from corrosion, but they should be perfectly usable. The cylinder bores (the most expensive part, naturally) are probably a write-off. I can feel significant roughness in the area where the o-ring rests, so I'll be very surprised if they don't spray brake fluid all over the rotors.
After a few days doing landscaping, I got back to scraping Poly-Tack and Poly-Brush from the fuselage. Apart from some small areas where the frame is sitting on the saw horses, the scraping is mostly done. Next step will be to wipe it all down with acetone to remove any remaining residue. Once that's done, I'll need to inspect every inch to find places where bare metal might be exposed, either from inadequate powder coating from the factory, damage during storage or transport, or my own ham-fisted scraping efforts. Those spots will need to be cleaned of any corrosion, roughed up with light sanding, then sealed with fresh epoxy paint.