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You could likely anneal it if it was out of the casting - but it will
likely cool too quickly in the casting.

It's worth a try! Spot heat, using a tiny torch tip, then keep applying
heat intermittently, slowing the cooling. If there's seals near that need
protecting, a wet rap wrapped around the shaft near the seal will do
wonders. Likely not an issue, anyway, and there's countless numbers of
seals on the market, one of which may work perfectly well, or can be adapted
to work just as well. In spite of the claims to the contrary, I have no
expectation that heating the screw won't anneal it enough to permit
drilling.

I also endorse the EDM idea, but that may not be a choice.

Someone mentioned the use of a left hand drill. Great idea. Heat first,
then try drilling. Fair chance the drill will bite and remove the screw.
Stranger things have happened. If it works, it will happen so fast you
won't believe your eyes.

Heating in and of itself will produce results. If you can heat the screw
hot enough to degrade the rust, most of the problems will have been solved.

Harold