The main thing that needs to heat up quickly is the catalytic converter, not the engine. A block heater doesn't get anywhere close to hot enough to make the catalytic converter start working. By the time the cat is hot enough, the engine is plenty hot from efficient direct burning of fuel as opposed to inefficient electric heat.
There's also a more practical concern. The Prius computers carefully monitor current flow in the battery. Any unexpected flow will cause them to disconnect the main relay because they consider this to be a hint of dangerous conditions. Nobody's discovered how to reprogram the computers (at least nobody has reported doing so on the net). Since the computers are in charge of so much safety, it's probably not a good idea to try reprogramming them anyway.
The computers are also responsible for carefully maintaining the charge level in the battery to extend the useful life of the battery. So even if you pulled power out of the battery in a manner undetectable to the computers, it would throw off the power maintenance which could seriously shorten the life of the battery. It would also void the warranty, so you'd have to pay for the new battery.
There's also a more practical concern. The Prius computers carefully monitor current flow in the battery. Any unexpected flow will cause them to disconnect the main relay because they consider this to be a hint of dangerous conditions. Nobody's discovered how to reprogram the computers (at least nobody has reported doing so on the net). Since the computers are in charge of so much safety, it's probably not a good idea to try reprogramming them anyway.
The computers are also responsible for carefully maintaining the charge level in the battery to extend the useful life of the battery. So even if you pulled power out of the battery in a manner undetectable to the computers, it would throw off the power maintenance which could seriously shorten the life of the battery. It would also void the warranty, so you'd have to pay for the new battery.
