is the ivory off the keys of an old circa 1900's piano worth any value?

Actually, white piano keys are not made entirely out of ivory. They are blocks of wood covered in sheets of ivory veneer. Same with the ebony black keys. These materials were very expensive, so no piano manufacturer would have wasted solid blocks of these when the thin veneer sheets would do.

A set of those keys would have more value to someone who could use them to restore an old piano.

I'm sure they are, they could be carved into small ornaments and sold on.