Looks like I may have solved it using a combination of Pterobyte's great advice in the forum, the mailbfr tool from Topicdesk and some creative cut and paste.
1. Stop the services.
2. Move the offending mailbox folder to the desktop (after permission changes) didn't delete it right away in case I needed it later.
3. Opened the hung client (offline) and copied the messages out of the offending folder to a local "On my MAC" folder.
4. Ran mailbfr -m username again and this time it succeeded (it restored the offending folder, empty) . Followed it up with the recommended mailbfr actions to fix the quotas and permissions.
So far it appears to be working.