with arduino, you could connect directly from the arduino to the MPC via midi and jj's new feature.
however, in order to maximize the 2 midi inputs i have i'd route it to my computer first, through reaper, and then back out to one of the mpc w/ other controller data. this probably doesn't count as "without other software" but it's a bonus because i can use reaper to manipulate, translate and record navigation button presses on my mpc. think about it - recordable mode changes, perhaps automatically changing your tracks for you as you play different pgms in a live set.
http://www.arduino.cc/playground/Main/CapSensei'm not using touchpads, that's the best part. with capacitive sensing, you can use anything that's metal, as long as it's got the proper resistor pairing. i'm actually using a taken-apart, 1st generation mpc1000 pads -

cover the metal coil with tape, plastic, rubber...and then touch the surface material, and the receiving pin sends a value. i was testing this earlier with the 1k OG pad and it was awesome - sensing my finger touching 2 layers of scotch tape taped over the copper coil on contact at 900K ohms. more surface area yielded more value.
no moving parts, nothing to break. properly grounded and conditioned, this could be an extremely sensitive and quick-reacting touch surface.
btw, don't get caught up in looking at different arduino models.
just go here:
http://shop.moderndevice.com/products/rbbb-kitand get that fellow or a kit for 5 + the usb bubb to program them.
saves a lot of money. when i start making midi controllers for people i'll be using these. the only downside is that you need the bubb to program it instead of a direct usb cable, and also it doesn' fit shields (but shields aren't all that anyway)