Here is an old trick that some of us use on antique auto screws that are frozen.
Materials needed:
8 volt wet cell battery (auto battery)
small gauge wire with alligator clips on them (like jumper cables only smaller)
How to use:
Attach clips to battery
Attach distal negative clip to receiver
Touch distal positive clip to screw head

This will cause the screw to heat up quickly and when it cools should be able to be easily turned.

Hope that this helps.
Best,
John

Humble member of the League of Extraodinary Gentlemen (LEG). Joined 14 March, 2006. Member #1.