Mesa used to (back in the 80's) make a product called the Midi-Matrix. I used this for several years to do just what you talking about, as well as switching channels on my Mark III when I had it racked.
I think Digital Music Corp. makes something comparable if not more powerful called the GCX Audio Switcher:
http://www.voodoolab.com/gcx.htm
Combine that with a midi pedal and you should have complete control over your effects and switching needs.
Chris... 8)