Why not just use Encode360. I know that's not the answer, but I did look at WME assistant - but gave up when I saw the price. Some guy recommended it over at AVForums though as being quicker than Encode360.
As an aside Encode360 works very well, but damn it takes a hell of a long time to convert - upto or over 24 hours on my rig for a DTS 1080p file, but you don't lose anything and the output file plays great on a 360.
Some threads here mention it:
http://www.avforums.com/forums/search.p ... d=13433476