This wont be possible with an xml file afaik, the data is mixed up to much for it to be possible without extracting the parts from the .mesh and writing a completely new file.
Indeed you are right Casuir,

Working from an Export of an Export is unfortunently not possible the XML way :( as I have come to realise in some tests and recently got it confirmed aswell.

So your efforts, KnightErrant and GrumpyOldman are the best way to go about it :)

Go! Go! Go! hehe :)