I've got some time to play with XStreamDB now, so I'm working on the EOAdaptor that King started, trying to flesh out the functionality for use in CAREO/Extreme.
The fun part will be mapping EOQualifiers normally intended for SQL-ish things into invoking XQuery-ish things... It may not be difficult after all, but there is definitely a learning curve in my near future.
It does have a good "code smell" - letting EOF do all the heavy lifting involved with caching, locking, sharing the records, and having the code that invokes the queries be relatively transportable to other technologies (once appropriate adaptors are written).
Once the EOAdaptor is in place, it will be extremely useful for driving all kinds of XML-based applications (the LC website would probably be overhauled to use XML, too, and since we're talking frameworks and adaptors here, it probably wouldn't even be a major undertaking)