I don't know of any hard-and-fast rule anywhere about donor credit lines
in exhibitions, but there is a strong TRADITION of providing clear and
specific credit lines, especially in art museums, and I heartily endorse
the system I have seen in all art museums I've encountered (I'm sure
anyone with contrary experiences will jump in to disagree). Even in the
case of loans which are expected to be converted into gifts later, I've
seen the credit, "Promised gift of..." Such policies need to be made
clear to donors and lenders, who have a right to expect consistency in a
museum's policy. I don't know how many museums include in Deeds of Gift
or comparable agreements language about credit lines, but I would
recommend that credit lines be negotiated up front. This also gives a
donor an opportunity to request anonymity in credit lines. Many museums
give credits for all items, whether on display, in published catalogs,
or in catalog records available for consultation by the public, viz.:
"Gift of...," "Loaned by...," "Promised gift of...," "Museum purchase,"
"Anonymous gift," "Acquired through gifts from...," "Provenance
unknown," etc.

I think it's important to explain to donors and lenders that your museum
cannot guarantee display of a gift, yet a museum ought to be able to
guarantee appropriate credit lines as thanks (and enticement) anytime
exhibited--or just in the records if it is never placed on exhibit,
while the donor's privacy receives reasonable protection (the museum
won't supply addresses without permission). The donor should be able to
request permanent anonymity if desired--or be able to change the credit
status when necessary. I like to be able to maintain a consistent stance
in this area with the reasonable expectation that your successors will
respect understandings and agreements you negotiated. I was recently
chagrined to lend collection items to an exhibition which did not give
credit lines, and felt that the donors' trust had been violated.