Why not just design the house to use such little energy that supplemental heating is very minimal?
I utilize what you are suggesting when I need to work on my car in the winter, make sure it is nice and warm before pulling into the garage for the free heating.
What we really need is an exhaust heated oven to cook our dinner on our drive home. Simple heat exchanger... Or how about a engine coolant heated crock pot.
My issue with the phase change material is that they tend to loose their effectiveness over time.
Toyota had the vacuum insulated canister to store heated coolant to preheat the engine for the next trip, it would be cool to see that idea on steroids.
It frustrates me in the winter to see industrial cooling towers running expelling waste heat from industrial process water, and then to see the same plants burning natural gas to heat water for building heat. Not even trying...