Did I ask for this before? ;}
If would be helpful if, say, a vaSlab would take a surface that is part of a brep as input (selected with CTRL-SHIFT).
Same goes for all the objects that need lines/curves as input. Command asks for a line - CTRL-SHIFT select an edge and be done. Rhino commands support this.
This idea came up since I have a mass model of a building (stacked closed breps) and want to build it up quickly with VA objects.