I know someone that did this with an old Prius. No software hacking required. Just leave the ignition on, and wire into the high-voltage system (not at the same time, of course!). The engine auto-starts when the battery gets too low.

The more interesting approach is to do it with a used EV. You can get a 100-133 kWh car used for $20-30K. The last time I checked, that was below the price of a house backup battery with a quarter the capacity.