Well, in testing, I was able to successfully remove a test user from MSOL (then the recycle bin), which deleted the test user's email. After running a dirsync which created the test user, and then applying proper licenses, it re-linked the test user's Sharepoint and Onedrive, while creating a new blank mailbox.
I'm not sure if this is the "proper" way, and I don't know the retention time of the orphaned Sharepoint / OneDrive, but it worked...