Too bad our government took "warranty void when towing" as "towing is dangerous", and as a result:
If you tow anything at all with a Prius, your insurance will no longer cover you, and you'll get pulled over to leave the trailer at the side of the road and get a huge fine.
The only legal way to get a Prius to tow anything here is have it tested, this year they added a new procedure so you can finally have your car tested for higher tow weights, they add weight to make the car full load, then add a trailer of the desired tow weight, then do 5 uphill starts and 5 downhill handbrake holds, if the motor/gearbox and brakes don't blow up, you get the tow rating..
And that seems like an excellent way to fry the MGR2, so I don't want to risk it, really.. And thats why I haven't bought a Prius yet, I still want one, if only for the fact that its easy to convert into a PHEV.
As far as towing trailers with small hatchbacks:
My 1600 lbs hatchback has an 800 lbs unbraked and 1400 lbs braked tow rating, so I used it to move house. The huge air drag nearly doubled the fuel consumption, but it was still a lot cheaper than renting a van.