Shimano Di2 Electric gears are a great innovation. I use 2013 Ultegra Di2 but my Zipp Vuka bars have integrated levers, so I couldn't go with the Shimano integrated brake/gear 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 rear-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. I used Cateye Waterproof switches (CD300DW), 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 and used another pair of Cateye switches, 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.
I tried switches on the brake lever, but migrated away from that. I prefer the switches better hidden, under the end of the base-bar, which is still very easy to press.
Final part of the project - hiding the original Di2 battery (SM-BTR1) in the downtube via the BB30 bottom bracket. By trimming the rail on the battery, and then inserting the battery and shroud separately, this can be achieved, albeit with difficulty. The assembly just about fits into either the down tube or seat tube of a Jamis Xenith T2. I hard wired a cable into the battery assembly so I can charge is simply by removing the seat-post and plugging in. Admittedly, using a Jamis Di2-specific frame would have been easier, as would using a Shimano internal SM-BTR2 battery.
Original switch unit, taken out of the switch housing and ultimately housed inside the right aerobar:
Switch housing with replacement switches:
View of the BB30 Bottom Bracket, with end of SM-BTR1 battery just about visible
Modifying the battery charger