So any updates on this project?
I just bumped on this thread while going with a similar idea. Just in my point of view storing energy wont really make a big difference and its a hell of a nightmare (such as engine block heaters, insulation...) If the car is left 12H there could be as little as half of the stored energy available. After a weekend, there's virtually nothing available. A remote tank reduces the available coolant flow and increases the warm up time.
I'm using a fuel powered (diesel/gas) heater to quickly bring the engine to temperature. a 5KW version will bring the block to a decent temperature (40-50C) in 15 minutes or so, using about 100-200ml of diesel fuel. At this point the limiting factor is the lambda sensor warm up, which should take no more than a minute.
Here's one:
The only mod required is the fitment of an electric circulation pump, to circulate the coolant with the engine off. Some burners have one already fitted, some don't.
No additional coolant and no expansion issues.