Well, I agree that Di2 doesn't do that right now, and their current market wouldn't accept automatic shifting per se. However, Dave N finished his query with:
"If not, surely this kind of thing should be the next step?"

IMO, if the mechanism is reliable*, then all that's really needed is the software to control it. Between Fairwheel and Shimano themselves, the software already exists. Integrating it to the level of a marketable consumer product is another kettle of monkeys entirely.

*The first time I got on a demo Di2 bike on a trainer, the front derailleur jammed in between the two chainrings. The Shimano rep was watching me, so I wasn't trying to do anything funny. When I got off, he fiddled with it for a couple minutes, then rolled the bike behind a curtain.