Hack the Keytar
The plan is to build a MIDI IN circuit as described in the MIDI specification, wire an XBee up to its output, stick it inside the keytar, and run wires from the back of the keytar's MIDI OUT port to the input of our MIDI IN circuit. In this step we'll open up the keytar and splice into some convenient power wires and solder wires onto the back of the MIDI port.
Remove all the screws on the back of the controller's case. There are four that are shorter than the others that screw into the back of the keybed - keep track of these and where they go. Flip the controller over, separate the two halves of the case a tiny bit, then slide the top half forward and around the front of the keys, then tip it back and clamshell the whole thing open as shown in the image.
We'll use the VDD1 and GND lines running to the touch sensor to power our circuit - we can get to them easily without taking the whole controller apart (although check out the teardown in the appendix if you're curious), and there's some free space nearby where we can stick the XBee. I clipped my multimeter probes onto these wires, and they seem to carry a nice, steady 3.3V no matter what I do on the keytar. This might affect the range of the XBee a bit, but since power is provided by 3 AA's (3 × 1.5V = 4.5V max), it's my guess that everything inside the controller runs on a regulated 3.3V so that the voltage stays steady. So, that's probably as good as we'll get, but hey, low power is the new something something.
You can see which lines are +3.3V (VDD1) and GND by reading the silkscreen (see image). Use a razor/knife to slice about a 6cm length of these two free of the plastic ribbon joining the five wires running to the touch sensor PCB. Then snip them in the middle and strip the free ends. Slip a bit of heat shrink onto one of the wires, then solder a new wire to each pair of wires you cut. Slide the heat shrink over the bare wires and shrink it down to keep things from shorting.
Now for the MIDI port. Solder a couple of long wires to pins 4 and 5 (the pins on either side of the middle pin, see image). The wires should be long enough to reach around the top of the keybed and down to where the XBee/MIDI IN circuit is going to sit, just next to the touch sensor. I recommend lots of solder and a hot iron to get a good bond, and then securing the two wires to the PCB just behind the port with a bit of electrician's tape, to reduce strain on the solder joints and keep them from popping free.