The only thing that would concern me is thermal shocking the engine. There would need to be a mixing system of some sort. I wouldn't want to have hot water shot into my cold block and head.
I could be wrong, but from the way I understand the article it sounds like it only heats a small segment of the coolant while it sits idle.
That would get quickly diluted when the engine starts, and then the thing would evenly reheat the rest of the coolant, right?
But yeah, cracked heads and blocks would be no fun.
Prius has a thermos bottle with coolant in it that gets pumped into the block when you start up - that is the whiring noise you hear when you take off on electric power before the engine starts up - it saves the engine hot coolant when it shuts down the same way. I think it holds 3 gallons.