I don't know what you mean by "pre-choke" but MegaDrum sends MIDI Aftertouch 127 when the edge/bell switch is pressed and MIDI Aftertouch 0 when the edge/bell switch is released.  See Chan 1 through Chain 16 Polyphonic Aftertouch in 
http://www.midi.org/techspecs/midimessages.php  . Same as Roland/Yamaha modules do.