Updating this thread.
I rebuilt the right rear caliper today. The piston was really tight in the bore. I pinched off the brake hose and disconnected the hose and got the caliper off the car. The piston threaded out of the caliper body quite easily, and the external boot, which was torn, came out easily.
The piston had light rust but cleaned up well, and the caliper bore was perfect.
The worst corrosion was where the external boot seal with the caliper body. I cleaned this with diligence but I worry about the ability of this boot to seal against the caliper body. The square section O-ring was clean and its groove was is good shape.
Reassembling the components with new seal and boot, and bleeding, was without any difficulty.
But in the final analysis, I think it is likely that sometime in the snowy and salty season, a remanufactured unit will be the correct long term remedy.