TG - I had thought the same as you about the Di2 FD being necessary for the system to work. But I recently discovered that this is not at all true, the derailleurs work totally independently; you can run the system without the FD without any problems.
There was a cyclocross bike setup with only one chainring and a Di2 rear derailleur that was profiled on one of the bike news websites a couple of months ago. I wrote in the comments about wondering how they had got it to work without the FD, and the team mechanic responded that it was absolutely no problem and Shimano have no issue with this setup. You learn something new every day!