Hey Chip, So what you can do with the MP-1 is map an external prog change number to any MP-1 internal patch. E.g. press 25 on your foot controller and map that to any MP-1 internal prog/patch say 42. When you do that mapping, change 25 is sent out the MP-1 midi thru but 42 is sent out the MP-1 midi out and the MP-1 selects patch 42. However, in your situation it may not be particularly helpful. What would be better would be if the GP8 could perform mapping, but it doesn't (as far as I can tell). The way I use mapping is by mapping the FX device(s). So in my Quadverb, I map a bunch of MP-2 programs to the same QV patch (which is slight stereo delay and reverb) so I apply this Fx to most MP-2 patches where I don't need/want any other Fx. The QV (and many other rack Fx) have the same midi mapping feature as the MP-1/2 so they (fx) stay at the same patch when I change MP-1/2 patches.
There are some things you could possibly do with the X15, e.g. in Effects mode make the various buttons change things in the GP8 (turning fx or various parameters on/off). Other foot controllers (e.g. the behringer FCB1010) can do lots more, like for any given switch, you can send a bunch of different commands on different midi channels.
For what you have, it may be best to copy the same fx patches to the various GP8 locations you need them (as was your initial approach).