At least in MN, and I suspect it's the same everywhere in the US, if you're coasting on a public road you're doing it illegally. End of story. Will anyone ever catch you? No. If they did would you get in trouble for it? No.
I'm no mechanical engineer and no Prius expert- does the Prius disengage totally, or does it keep one driveshaft connected? I imagine it would have to keep something on to comply with state laws.
PS- Dave said we are hot rodders. I would like to propose that while fast cars are hot rods, our efficiency machines might be "cool rods" because, well, efficiency is cool.
