Assuming you're running the newer 10speed stuff (not the 79xx series)...

The 6770 Di2 shifters *will* work with 11-speed cassette/derailleur - keep in mind that the shifters are really nothing more than a micro-switch that sends a "move this way" or "move that way" signal to the CPU (SM-EW90). So, it really doesn't care whether the brain (and thus derailleur) are 10 ticks top to bottom, or 11 (or, presumably 12).

I can confirm because that's what I have on one of my bikes.