ok this is very possible, even Bjork is using an MPC as the sequencer on stage, connected to all the midi devices. perfect and stable

You will need 2 midi cables: JP OUT > MPC IN and MPC OUT > JP IN

Then you will need ot use the MPC sequencer.. I dont know how it works, so I will just guess: create a new track, make this track control an external midi device, and set it's MIDI channel to 1. You should be able ot select the instrument from the MPC (bank number, patch number) and if you hit the keys/pads on the MPC you should be able to hear the sound of the JP.

Maybe someone who has an MPC could post some more information here.. would be great