I think you gotta just test it over and over. I was trying to find a way to do it like how someone else did their scabbards in Oblivion.
For some models they had two parts for the scabbard: One was gemstones in the mesh, and the other was the scabbard itself. They were labeled SCB:0 and SCB:1
When you pulled out the blade, the Gems and Scabbard would stay on the hip.
I tried doing this with Skyrim and separate attaching belt. So that I could move the belt around and reposition it however I wanted, but it crashes the game lol. Maybe I'm doing it wrong?
I wrote a thread on it at niftoolsforum, waiting for throttlekitty to see if he can get it to work:
http://niftools.sourceforge.net/forum/viewtopic.php?f=24&t=3440&p=23197#p23197
You can download my files there to see if you can get it to work better for you.