Hi,
I was looking through the feature list in the issue tracker to see what's in the pipeline. I suddenly remembered a feature from an xml database I used a couple of years ago called Qizx. This had a very neat feature where every database document and collection could have a special map with metadata properties. These do not affect the XML content in any way but they can be accessed via special API calls or Qizx specific extension module.
A better explanation of this feature can be read in the Qizx manual (for example here http://kiwi.emse.fr/DN/qizx-manual.pdf on page 18 and 57).
I have used such metadata properties on nodes to implement syncing XML documents in a SCM (Subversion). I stored revision id's and other SCM control data in those properties. Authors would work in Subversion and certain directories where kept synced to a Qizx database so we could easily create PDF publications of the latest XML with zero impact on the XML itself.
Maybe BaseX already uses something like that under the hood, I don't know. If so extending it or opening it for use would be useful I think, and generally cool :-)