pluggable storage layer for Xindice?

[Gary Hallmark]

Do you have any thoughts, requirements, (or code) for plugging in an 
alternate storage layer, say an RDBMS like mySQL or Oracle, in place of or 
alongside the filer/indexer layers in Xindice?

Some benefits might include
- transactions
- online backup
- pushing some xpath subset down to the storage layer for evaluation, if 
supported (Oracle9i supports an xpath subset, e.g.)

Where would be good places to "plug in" to Xindice, and what are the 
potential gotchas?

Thanks!