If you tapped the traction batteries' output to an inverter there wouldn't be a way for the Prius to realize it wasn't operating normally with its strategy to keep the pack voltage in a ~30%-80% state of charge. It doesn't have a particularly large battery for the traction battery so I'd imagine it would run a lot unless you tie it to a larger pack. There are some folks retrofitting LiFePO4 cells with individual charge controllers to trick the NiMH hybrid system into using them. All the car controls look at is cell voltage.