DA Di2 uses direct hard wiring for power and signals. Ultegra Di2 uses CamBUS comms protocol. This relies on a component I.D. signal, allowing the electronics "BUS" to correctly interpret and communicate with the component at the other end of the cable. Which is also why it allows for dummy-style connections where you can connect in any combination at the harness junction box and the system will still work as signal processing is by ID of component and not by specific connection points.
However, CamBUS will prevent you from so easily hacking the battery and extra buttons simply by just soldering extra connections. Since an ID is required for each component to communicate. It still can be done, but a much higher level of electronics comms knowhow is required. So, you're stuck with a standard setup unless you don't need those 'extras' at least for now. I believe we will see those auxiliary buttons and switches for Ultegra Di2 when DA eventually switches to the CamBUS protocal in about 2 years(when its up for revision). For now, probably not yet as I'm sure they don't wish to cannibalize their own products just yet.
Note, for current DA Di2, you can quite easily forgo the custom battery and charger by opting for a hacked battery pack using min-RC battery packs and chargers and saving a decent US$200/- or so in the process. Likewise, its possible to add additional shift buttons for under US$50 with some tinkering and soldering as demonstrated by some forum contributors here and over at Fairwheelbikes forums.
Some stuff to be aware of in the overall decision of things.