I towed around a car (not 911) on an open car trailer behind a vintage motor home and I would not recommend it. Way underpowered, and there are many places we stayed where that trailer was a huge liability. Added stress to the journey, and if I were to do it over flat towing the "dingy" would be my choice.
Some people I met and befriended on my RV journey were a family of 5 who were full timing in an older Winnebago. They flat towed a VW thing all over the country for years, never seemed to have any issues with the trans. VW thing <> 911, but I'd expect they are pretty similar
To address any concerns about oil circulation in the trans, couldn't you install something like this:
http://www.patrickmotorsports.com/part/g50-930-915-911-901-transmission-oil-cooler-system-trans-gearbox/
You could wire up the pump so that it could be plugged into a 12v source from the motorhome to circulate the gear oil while flat towing.