Revit :: Negative Joint Mullion Curtain Walls

I'm trying to get a curtain wall style working, where I would have panels with a clean joint between them, of a controllable size. I'm also working with system curtain wall panels as I expect the CW style will need to work with variable profiles at the top.
 
My attempt so far has been to do a U-shape profile to get the panels to stop against, but this means the U is seen, particularly at the + junctions of the panels, which is not desirable. I could locally hide the mullions, but this is time consuming and I'd rather do it using a style-based method.
 
I've also tried doing a filter based on the CW Mullion name, and making it not visible, but this doesn't work for some reason.