[Al Skierkiewicz]

A relay is the way to go here. There is no control of the power pins on the RC. and the solenoid would draw current that could exceed the capabilities of an RC output. A simple tristate interface would not handle the current from the solenoid but a high current hex driver might. You need the current specs from the solenoid to determine the best course of action. Don't forget that the solenoid will generate a rather large voltage spike when the power is removed and the device driver needs to be protected from that abuse.
Yes the PWM connections on the RC are tied to +7.2 volt backup battery (in 2004 and later) and as such power is not available without a battery connected.