This came up a couple years ago. I was told by an insider that someone could create an HDi program that turns any HD DVD player in to a media extender. You would have to put the program on a disc in order to run it.
From what I remember, it could play any file video that used the codecs supported by HD DVD, such as mpeg2, VC1, DD+, Dolby TrueHD, etc. And IIRC, the server could be on a home PC or somewhere on the Internet.
Maybe someone can ping an old HD DVD insider like Ben W. or Amirm and verify this. I think the only problem is that you can't just find a free HDi development kit anywhere.