Just keeping this for my own records, but if anyone else finds it useful, tally ho!
Museums with Open Access collections:
Art Institute of Chicago; filter by “Public Domain”
Biodiversity Heritage Library: on Flickr, navigation is a pain
Cleveland Museum of Art, Open Access page
Indianapolis Museum of Art at Newfields
Los Angeles County Museum of Art (toggle public domain images on)
Yale University Art Gallery – check individual entries for copyright status
Yale Center for British Art (“Free to Use” selected)
Rijks Museum – only displays public domain results in image search
Museum of New Zealand – search “with downloadable images”