Currently you can only have a curtain panel go into (recessed in the frame) or in front of a mullion if you are using a system panel. If you need to use a custom panel, then the panel will always use the left/right edge of the mullion bounding box to define the width of the panel.
This is a frustrating limitation. It would seem that a minor tweak to the way the widths are calculated and some extra reference planes in the template could make this far more powerful.
EG: if the built in template had a Front & Back, Revit could then analyse the geometry within this box and use that to determine the width. EG: if that geometry fits neatly into a recess of the mullion, it would let it. OR if it can continue to the centreline of the mullion without clashing it would let it.
Equally, extending on this the same could be applied for the top and bottom.
This is fairly common practice and unfortunately the system panels don't always meet our LOD requirements.
