I have a Qt project which uses XML files. Those XML files contain human-readable text and this text should be translated by using the Qt tools (lupdate, lrelease, QtLinguist).>
You can translate anything you want at runtime by using tr(), as long as the .qm file has a matching translation/context. It shouldn't make any difference whether lupdate extracted it or not.
I don't know how to make lupdate to extract strings from arbitrary XML, but that doesn't mean you can't use linguist.
If you really want to have it all in one file for the translator, have your XSLT copy the lupdate-generated file into its output.
As long as you use a context name that doesn't duplicate something used in the source code, this shouldn't be any different (from Qt's point of view) from the way many apps load a .qm for each DLL that has GUI.