Has anyone ever heard of Documentum being used to provide the backend store for Subversion?
Haven't heard of this.
I don't think it is too likely, either. A subversion back-end needs to be a more general purpose datastore than it itself is and documentum is geared towards specialized storage and management of documents.
Someone could certainly shoehorn it, but at the risk of putting words in your mouth, it seems like you might be asking about something like a bridge where a documentum store could be presented as a subversion repository.
Not knowing too much about documentum, I would say that this is unlikely, since svn is based on the idea of directory based grouping of changesets and repository-wide versions.
A better match might be something like CVS, which is pretty much an aggregating interface for an underlying revision management system (RCS).
Though possible - doesn't make sense. Cause Documentum already provides some version management capabilities (similar to subversion). So it would be like having two version management systems stacked end to end.
I think whoever had this idea is confusing Documentum for a storage solution (like EMC Centera).