After playing with this for quite a while, I thought it would be a REAL problem. Girder methods may work but the gml that was posted in another thread, didn't for me. I think it may have been for an old version of Powerdvd anyways.
I have quite a few hd-dvd's on my hard drive and want very much to play them through Xlobby. I was thinking since .evo is the file format of hd-dvd's, that would be the extension to associate. Wrong.
In editing the .xpl file (hd-dvd's version of xml), I released that by launching just this file, it would in fact play the entire directory (read, the hd-dvd movie). So, I associated Powerdvd with .xpl, used "%parameter%" without the quotes. After you add the movie directory, it will be picked up as adj_obj, as that is the associated dir. Just go into the database and rename it whatever movie, and wallah, Perfect HD-DVD playback within Xlobby.
Hope this helps somebody.
g