This may sound silly, but is it possible to use Custom Persisters to abstract away any concrete form of persistence and then use the OQL to query against the disparate resources--whether they are ultimately persisted in a RDB, in a flat file, or whatever?

Thanks for Hibernate! When I've proven it to my company I'll work to
(at least) publicly give you guys credit!!!

yes, it is possible. You would need to implement Queryable (which is not trivial).

Hmmm, I've looked at Queryable, but I'm having trouble envisioning...
if going after say a flat-file implementation, would enough of the
implementation be specific to the fact that its a flat-file to make it worthwhile to get that far and then everything else is a customization based on the format and contents of the flat-file...maybe take it a step further and lay groundwork for an expected XML format so the customization would be mostly based on what's in that XML?

...wish I had time to go after something non-trivial...