Update: I was able to access the database and pull the username used by the emergency account: Passwordstate\EmergencyAccess
I don't see this documented anywhere in the manuals, so hopefully it can be incorporated and this post will help others. I'm including the text of the prompt below for search engine indexing:
SAML Maintenance Mode User Check
To finish the upgrade process, please specify the User ID of the Administrator who enabled Maintenance Mode for the upgrade. Please UserID in the format of Domain\UserName