I'd also suggest 3D-Mobsters approach, with constraining the dress to Roxie, where the belt sits on the dress and later then constraining the belt. The collision target of the belt is the dress, with a minimal offset as possible.

Then you first run the dress sim. and after that the belt sim. (if wanted/necessary)

I did tons of successful cloth room sims, with this approach exactly, using the dyn. cloth sets by this vendor here @ Rendo: http://market.renderosity.com/mod/bcs/?vendor=400165