A midi controllable a/b box would be your best bet. I looked into this option for when i was thinking about keeping my RM4 after I got my RM100.
Plugging guitar directly into the A/B box, then A into RM100 and B into RM4 & running the RM4 into the series return of the RM100 would have then given me 7 available channels all midi switchable as both the RM4 and RM100 have midi in out, i don't believe that the RM50 is midi switchable though? which may make things more complicated for you.
Anyway here is a link to a midi switchable a/b box that would do the trick
http://www.soundsculpture.com/products/pt_abcadabra.htm
Goodluck I hope it helps.