One helpful resource when researching rights-holders of objects is the MusIP Yahoo! Group. One has to request permission to join, but the group is helpful because it has immense archives, replete with Museum professionals across North America requesting information on certain artists, and in some lucky cases, receiving that information.
For example, when one searches for “Diebenkorn” amongst the listserv’s archives, one returned entry contains the address to the current rights-holder for Richard Diebenkorn. I recently posted a query regarding Czech artists and received both a helpful phone call and email.
This is one of many sources that you currently have at your disposal when conducting research on rights-holders.