no the two moves are similar in the sense that they are both inverted but are quite different, look closely, a back pan am is described in the international rules as-
Back pan am- Elevated 180 degrees rotation on green
water at an inverted angle greater than 90
a mc-nasty- Starting in a back blast, the boat is
bounced, inverted and the stern thrown into
the green water. The boat half loops back
upright so the paddler finishes the move
facing upstream.
a reverse pan-am looks more like a reverse air blunt only you invert the boat- a mc-nasty is more like a bounce to upside down to a loop...I'll try to find some pictures of a mc-nasty in a hole as that makes it much clearer