For my money, the electric blanket idea is a bust.  For one thing,
the entire piano would not be heated evenly: some parts would be hot,
some cold, and the condensation would be an additional problem.

I recently redid a piano that had been in an unheated garage here in
Vermont for 15 years, and the only problem with it was the mice damage;
the soundboard, pins, everything, was sound as a dollar while other
pianos ("well taken care of") come in with cracked boards, loose pins,
you name it.  If the piano has been fairly stable I'd recommend just
leaving it be.  After all the weather in the winter would be pretty
steady as to humidity and temp.

If it is a grand piano, though, I would definitely take pains to
protect against mice!  I saw one small grand that had been stored
for several years on its legs, instead of on its side, and mice had
made a "run" of the entire soundboard and keybed.  It was not only
disgusting but _very_ unhealthy.

Ana Gerard