I've written a little program for personal use that lets you drag & drop any file to your xbox & it will start playing automatically. Equally if its on a samba share it will pass the samba share link to it. Finally it also lets you copy youtube urls and it will play them on the xbox (resolves the file location automatically.) Its likely to barely work but if your interested find it here.
Drag & Drop xbmc
(i tried signing up to xbmc forum itself but never got a activation email anyone else have this problem? tried with a gmail and btopenworld emaild address.)