Time to share my Di2 mods, now with TRIPLE changer positions.
My thanks to Turboferret (Rich) for his thoughts, shared in this thread and directly to me.
My Zipp Vuka bars have integrated levers, so I couldn't go with the Shimano lever option, plus I often climb with my hands on the pads, so fancied having access to switches from there too. By pulling the electronics out of the bar-end shifter I can now change the front and rear derailleurs from the aerobars, and can also change the rear from both the drops and, while climbing, from the elbow pads. As others have done, I used the Cateye Waterproof switches, under $15/pair from Performance Bike.
I removed the electronics from the RD changer and brought my own wires out of the circuit board. My plan had been to use a circuit-track pen to reach the centers of the circuit, and then mask those tracks, so as to still use the original switch mechanics, but that was unreliable, so I just took three wires out of the electronics unit before sealing it. Note that I took the common wire to one of the pins on the right, but could have used either of the larger pads adjacent to the switches.
I put the electronics inside the aerobar (as TurboFerret did) and put a couple of waterproof microswitches in the Rear-Changer-Housing. We both found this wasn't very waterproof. His solution may or may not involve a condom, mine involved removing the housing from another pair of Cateye switches, and placing them on a crafted piece of plastic. They function perfectly. I added extra insulating goop, this project needs plenty of that, plus zip-ties, soldering, heat-shrink sleeving, electrical tape, and creativity.
We both tried switches on the brake lever, and both migrated away from that. I prefer the switches better hidden, under the end of the base-bar, which is still very easy to press.
Next project - hiding the original Di2 battery in the downtube via the BB30 bottom bracket.