Just closed this issue so mullions and any other children elements (for revit families like walls, floors, and roofs) should now be received correctly as meshes in Rhino! This will be available in our next beta release.
Currently, all children elements (like mullions) will be sorted into their own elements::displayMesh sublayer within the parent family category. This is what a basic curtain wall with mullions would look like, where the panels are in Walls::displayMesh and the mullions are in Walls::displayMesh::elements:displayMesh:
Let us know if this generally works for your process!