> If you don't mind using extensions you could use a combination of
> saxon:evaluate() and saxon:path()
> 

Yes, though there could still be complications involving namespaces.

I think I would be inclined to tackle this by doing a depth-first traversal
of both documents in parallel, writing the generate-id() values of
corresponding nodes to a third document, and then using this as a lookup.

Michael Kay
http://www.saxonica.com/