Mmm, just re-read my old MP1 manual, it isn't particularly intuitive. I'm more familiar with the MP2 which gives you a MIDI change table (x = x etc). So in section 4.0 MIDI Functions, it describes how to take any external change number (1 - 128) and map that to any MP1 internal patch (1 to 128). If you press PRGM button (active), the display will show the midi pc number being sent to it, say 10, to map that to a different internal MP1 patch press MEM, scroll/select the patch number say 55, press MEM now when the MP1 gets 11 on its midi in it will change to its internal patch of 55 (which is also sent to the MP1 midi out).
Alternately, you can do the mapping in the GMaj as it has it's own mapping table eg send 11 to MP1, (MP1 changes to 11), GMaj also hears 11 (from MP1 midi through) use the GMaj change table to map that to whatever say 60 (GMaj goes to 60) (or maybe 59, if it's a 0 - 127 unit like my Quadverb). For both scenarios the MP1 and GMaj can be set to omni (all 16 midi channels).