Hopefully we can allow multi-user access to any backend. The reason for the single-user restriction was to improve the performance of the BSDDB database.
If the _create_undo_manager approach doesn’t work then I am open to consider alternatives. It may also be time to have another look at storing objects as JSON rather than pickled blobs.
I’ll release v5.2.1 over the weekend. After that, I’ll have time to start thinking about v5.3.