To continue; Link is right. There is no such thing as a permanent loan — think about the words. They become bugabears and take great effort to untangle after a few years and staff changes. However, the decision seems like it is out of your hands. I hope there are not many and that they are on exhibit, not lollygagging about in storage.
I would suggest two accessions. One with the donation/gift. The other the loan (which is not permanent). Assign the next in line accession number and go to Incoming Loans and catalog from there. Will they let you mark the objects, or do you have to use tags. Write into the incoming loan agreement that representatives of the foundation/ loaners, must arrive once a year to inspect condition of the objects.