Any question answered...

Do you actually need Di2 levers for electronic shifting, or could you use a climbing shifter (e.g. SW-R600)?

Currently have a spare frame with 5700 shifters and only the rear derailleur cabled up. No brakes on the frame.

Thinking of putting an 11sp cassette onto the turbo so that I don't have to faff if swapping bikes.

Could I get an 11sp Di2 rear derailleur and a climbing shifter button on the bars, and just control shifting completely with the button, leaving the 5700 levers there without any cables connected at all.

(saw a comment on road.cc that implies it might be doable
http://road.cc/content/forum/213603-di2-climbing-shifter-and-r785-hydraulic-levers

The SW-R600 climbing shifter is a self-contained unit with an E-Tube connection and doesn't need to be connected via a lever. the Sprint buttons are just extensions of the main lever and won't work connected directly. So you can connect the SW-R600 anywhere there's a spare hole. A junction box should work okay; the 5-port control box is a more elegant solution but also quite a lot more expensive.

)