Is the XTR Di2 battery compatible with Ultegra Di2? I'm considering installing an Ultegra Di2 groupset on a small road bike.  The space below the water bottle bosses is too little to install the regular road Di2 battery, but the XTR SM-BTC1 Di2 battery holder looks like a good option: http://bike.shimano.com/content/sac-bike/en/home/components1/mountain/xtr-m9000/sm-btc1.html

The question is, can the XTR Di2 battery actually be used with Di2 road shifters like Ultegra 6770 or 6870?  Or is there some other battery offering similar mounting options for road bikes?

One reason for optimism is that the XTR battery page says "Six E-Tube wiring ports with cable organizer," which sounds perfect since the bike in question has no space for an internal wiring junction down near the bottom bracket.

The page also says it holds the SM-BTR2 battery, which seems to be exactly the Ultegra/Dura-Ace internal battery stick that goes in a seatpost.
According to BikeRadar, "The battery unit as well as the wiring for XTR Di2 are identical components to the ones used in Shimano's electronic road groups. The battery can be mounted on a bottle cage, in a seat tube, frame and can even be contained within the steerer unit of certain forks (via the use of PRO's new Tharsis components)."