"I believe there are sprays (GT85 PTFE based) for use on pedal cycles where you spray the Shimano mechanism after a wet or mudy ride that are very effective at keeping,chains, cranks, pivots from rusting. After a winter ride a spray avoiding the discs might be effective."
That field is complete and utter bog of wonk opinions on what should be used on any of the components, supplied by manufacturers selling "special" potions in tiny bottles starting at something like £8.99 for half a fluid ounce.
maybe a soapbox view, but they really are nuts in there.
Working on a lot of that equipment, I treat the calipers the same as these bikes as they also have zero weather protection for the pistons. Calipers with 2, 4 or 6 piston in aluminium, stainless, phenolic or titanium all are fine with silicone grease. Also it's imperative that the seals are free and retract the piston promptly as pushing any drag along in a brake is purgatory.
As you point out though, keeping anything clear of disc and pad material is primary as it stops them working very quickly. Even cleaning the surface with brake cleaner has quite a dramatic affect on brake bite specifically and you have to put them through some duress and heat cycles to bring them back to designed efficiency.
