We recently built up a P4 with Di2. The good news is that it not only works... but its F'in awesome.
The ability to shift gears from from both the basebar and the aerobars has been a major highlight.
Our mechanics were able to incorporate the battery and a lot of the wiring within the aero bottle by
cutting out the bottom of it. So at this point is not so much a bottle, but a battery fairing gizmo.
Easton Crankset with Sram 54t ring and Mavic Ti Pedals
Notice the wire entering the bottle
FD wire exiting bottle
Casing removed to show hardware
Top shot of modified battery mount and wiring
If you are in town for the Longhorn 70.3 be sure to stop by and check it out. You will be impressed
by the perfect shifting of Di2.
-The team at Austin Tri-Cyclist