I have heard from one person who has made the Mpx-Futaba lead and it worked. One of Britain's leading Mpx shops also shows a Futaba-Mpx lead on their website and it is plain pin to pin.
www.modelspot.com/mpx/mpxradio.htm The different voltage is not an issue. And yes, the uni switch makes Mpx use the same centre as Futaba.
Harry