nice idea. just some more thoughts to share:
you could in fact put all the parts to the server,
then have the client send a "list of needed parts" to it.
the server could put all the needed files into a *.mpd on the fly (which is trivial),
and then that result could get send gzip compressed to the client.
you could in fact put all the parts to the server,
then have the client send a "list of needed parts" to it.
the server could put all the needed files into a *.mpd on the fly (which is trivial),
and then that result could get send gzip compressed to the client.