I just strumbled across an offer from a company which recently adapted their power liftgate kits to the Model S.
They claim to have an DIY plug and play installation which integrates the control module into the CAN bus. Means that it should operate nearly like the factory one. You can open/close if by touchscreen, liftgate buttons or fob. Setting the opening height is also possible.
The sell it for the chinese market and want to check the international demand for this. Until 12/31/2016, they will give an 100 USD rebate to preorders what makes it 299 instead of 399 plus shipping in case they will gain 50 orders worldwide.
You may check it out here:
TL105 Power Liftgate Pre-orders – Autoease
Sounds reasonable to me and I'll give it a try. The chinese installation instructions show some quite simple work which should be done in a few hours max.