Just a quick update on this, in case anyone enjoys building their own stuff and you are handy with a volt-ohm meter and soldering iron. After some experimentation, I created a nice foot pedal extension for the Novation Nocturn. The end result is the normal Nocturn controller you can use as a hand-operated mixer/controller, with a cable that runs to the rigged fooswitch/expression pedal assembly. The whole controller rig connects to the computer/MainStage via the Nocturn's USB.
I bought a used Nocturn for $60, used an old Ernie Ball VP JR volume pedal (just about any old wah or volume pedal should work), 4 guitar momentary footswitches, and some ethernet cable. The basic steps are 1) the volume or wah pedal pot has to be 10k. Replace the existing pot with a $4 Radio Shack 10k pot if required. 2) build or buy a box for your footswitches. 3) open up the Nocturn, and use your voltmeter to figure out the contact points on the circuit board for the 4 rightmost pushbutton switches. Easy to do. Also determine the common ground for these (so you end up with one common ground for all the switches). 3) desolder and remove the Nocturn slide pot. 4) solder 3 ethernet wires in the cable to connect the Nocturn slide pot contacts to the volume pedal pot. Nocturn contacts look like (: .) and the pot center tap is the uppermost contact. 5) solder 5 ethernet wires to the Nocturn 4 switch "hots" and 1 common ground; solder other end to your switches, with the common ground wire shared between switches. I built my switches and pedal into one unit, and put RJ-45 jacks on the unit and Nocturn so I can connect the two using a regular Ethernet cable.
Now you have a low-cost "dual" MIDI controller: the Nocturn works as usual as a nice hand-operated controller, your volume pedal is now an expression pedal (use for wah or volume, effects, etc), and your foot-switches can be assigned to anything you want in MainStage. (You could wire up as many as eight footswitches, if you like.)
Please only attempt this project if you are comfortable working with electronics. This is a relatively straightforward and simple hack, but I don't want to be responsible for any buggered Nocturns! If you aren't comfortable with these instructions, just buy an off-the-shelf Behringer or Apogee or similar foot controller.
