The other xmp element I use is the Xmp.lr.hierarchicalSubject where text is stored like "Where?|Germany|Berlin, Who?|Family|myself".
In mediatomb I then can browse and show all photos made in Germany, made in Berlin,...
I know there is no XMP parsing code yet but nevertheless, would be nice to have. And these hierarchical Subjects in the lightroom xmp are getting more popular, even none-Adobe tools use them.