I guess one would have to figure out, if the reason it’s not allowed above a drop ceiling is only related to the smoke development of the cable then you might be on to something. But somehow I’m not convinced that this is the reason cause not all drop ceilings are plenum ceilings.
No idea but having bought plenty of plenum rated Cat5e and Cat6, I don't really think you'd want plenum rated Romex it wouldprobably cost more than MC cable.