I just had a block heater installed in my 06 HiHy by the dealer in Madison WI last week. They charged me $255 parts and labor, and I even gave them the online instructions you can find at PriusChat or other hybrid sites by user "efusco." And I had to take the car back one day later because it was leaking coolant and they had to tighten up the connections. But it's fine now. Little by little, we hybrid owners educate our service technicians!
And it IS a Toyota part, from Toyota of Canada. Have your dealer check at Toyota Associated Products. You can view the parts online, but you have to be a dealer to order from them. It's Pyroil Engine Heater 644, SKU: 001130021300644.
I got the same song from my dealer about not wanting to install it because it could leak, probably in 2-4 years, possibly with a catastrophic loss of coolant. They did admit that block heaters were different years ago and they hadn't done one in a long time. We agreed that they would check it for leaks when I bring it in for service (they'll probably charge me extra to look!), and that I'd take the risk. I asked other users on PriusChat and CleanMPG, and no one had much experience except "efusco," who installed his own and it's been fine for nearly 2 years.
But I have to admit that the dealer/service guys couldn't understand why I wanted this, as they still think block heaters are only for being able to start the car in very cold weather.
Good luck - I'll be interested in hearing how you do with yours!