if they were my pedals I wouldn't give a toss if the binding parts started to go rusty; it makes absolutely no difference to the performance of the pedal. The worst rust occurs on unused pedalbindings that corrode where the plating has already worn off (which doesn't take long IME).
However what is important is that the springs are not allowed to go rusty; this can cause them to fail.
These screws are notorious for being right sods to undo; better that than they come loose though; I think they are factory fitted with threadlocking compound. They are JIS type screws IIRC, and only a small subset of phillips #2 style screwdrivers will do.
My suggestion would be to
a) wire-brush the steel parts of the pedals to remove superficial corrosion
b) coat the springs and the undersides of the steel claw parts with waxoyl.
c) Run it in using a hairdryer to melt the waxoyl, so that all surfaces are nicely coated with a thin layer; it will run into all inaccessible places.
d) wipe the excess off the exposed surfaces of the pedal
e) Ride happy.
f) repeat as required eg at 18 month intervals.
If you do start removing parts from the pedals, it is essential to replace the screws with some threadlocking compound, else they will work loose in service.
You can buy the correct screws from shimano. IIRC they are a standard M5 thread, with a csk head form, but the head size is non-standard.
You can make suitable screws using slotted head stainless csk M5 screws. The head may have to be machined down and the slot recut. I have an idea that the socket in a torx spec screw may be deep enough that it will allow the head to be reduced in size whilst retaining the torx function, but I have not tried it. XTR SPDs come with different (hex socket or torx, I forget which) headed screws, which also fit the other SPD pedals in the range.
hth
cheers
