Michael - First off thank you very much for using Office 365. You are correct in your investigation and understanding of resource mailboxes. The automatic booking assistant does not support booking a room for a meeting that occurs in the past.
This was the initial design of the resource assistant, I have opened up a Design Change Request. As part of each version, we review the DCRs and determine what set of features and design changes will be included.
Thanks for your feedback, and please keep it coming!
David Los
Program Manager - Exchange Calendar Team