Since I could not wait the few more months for the TT version of ultegra di2 to come out I decided to try and make my own. Long story short, it can be done fairly simply. Ultegra di2 uses canbus protocol for communicating between the derailluers, shifters, and even the battery mount. So, in order to make tt shifters, I used the STI shifters for its parts. The STI shifters have a small circuit board connected to the x/y shifter buttons on the paddles. The switches on the sti shifters are simple momentary switches with a common and up & down wires (three in total). All that is needed is the small circuit board and the wires coming from it to the paddles. The x/y buttons can be replaced by any momentary switch. For my project, I mounted the circuit board and switches into a section of the aero bar. Satellite shifters can easily be added in by splicing into the new switch circuit. From there, installation for the rest of the components are the same.
A few things I found....the harness is only made up of two wires so if youre good with a soldering iron, you dont need the junction box or 4 separate shimano wires...you only need the plugs and the rest can be made. Additionally, since even the battery mount communicates with the rest of the system, you cant simply put a battery inline like with dura ace di2. You can, however, use your battery of choice and just run the leads to the mount. I placed the mount under the seat and put the battery in the seat post.
It was a fun project and fairly easy. Ill post some pictures below!!