[herr_oberst]

I'd squirt some Kroil or PB into the gap, then wind as much waxed dentil floss as would fit in between the head of the screw and the plastic. (Along with pinching the plastic post with. pliers or tiny vice grips.)

Then, carefully back and forth the screwdriver in tiny increments to break the galvanic bond.

One thing to remember, too, those screwheads are JIS, not Phillips, so if you have access to the proper screwdriver, be sure to use it, just to give yourself a fighting chance.