Most collections in museums are held as public trusts, and are subject to use
in legal matters.  My only comment is to suggest that, whenever possible, you a
sk the people with the subpoenas if a notarized copy (of a tape or a blueprint
or a letter or such) would address their purposes.  Sometimes it does, and then
you don't have to try (sometimes in vain) to get the items returned.  Evidence
often does have to remain within the court's control.