There is a product that guitar repairers use called "De-Glue Goo"
which the maker claims will dissolve old dry hide glue.  It costs
$9.50 a bottle and can be bought from Stewart-MacDonald in Athens,
Ohio, USA: http://www.stewmac.com/ 

I have no connection with that company and just pass this on as it
may be of use to player piano rebuilders.

Don Watson