Yes. You can't put it into READY mode (ie hold brake and press power button), but you can put it into ACC1 and ACC2 modes to listen to the radio, open the windows etc.
FWIW the radio is powered off the 12V battery anyway, which is charged by a DC/DC converter from the drive battery when the car is in READY mode (or at 2pm each day if you have wifi enabled and the car is off and locked).