In the end, it depends on how one wants to use a
transcription file. The XML can be generated
automatically from a straight text file,
just like GC's PDF file is a product of some tool
that reads something from either an ASCII file
or some kind of database table.
The requirements coming from the desire to
visualise the page in VMs-lookalike form
are completely different from the requierements
to allow automated text processing.
So it is just my old-fashioned opinion that the
'meat' is the transcribed text and the file format
is 'dressing'.