An LPG conversion on a hybrid would be awkward. The engine's cooling system is normally used to heat the gas up in a device called a vaporiser. In the case of a hybrid, this might need some assistance in the form of an electric heater, and I don't know if these are available. If successful, the gas ECU would have to be reprogrammed so that it would both start and run on gas (the default on my Romano is to wait 25 seconds or until the engine reaches 25°C, this opens the tank-side solenoid, then wait for the engine to reach 1600 RPM, this opens the engine-bay solenoid and the system switches to gas).
Besides, running a hybrid on LPG? That's just tight! Still, if it claws back the money shelled out on an RX400h...
You are correct. The petrol tank stays where it is (although in some vehicles they can fit a smaller petrol tank to make room for a bigger gas one). If you run out of LPG, the system will switch back to petrol automatically.