basically, all you have to do is get rid of the faces you don't want (either through vertex shrinking or hexing them out), then changing the bone index of the face you want to keep to a different index (that way, it will always show).
here's a video on how to hex out polygons, if you choose to hex them out rather than shrink them down.
http://kc-mm.com/?p=2106and here's a little guide for the second part i made.
http://forums.kc-mm.com/index.php?topic=31231.0