You could always tap into the AC feed into the rectifier and use a seperated bridge rectifier to convert the feed to DC to power a relay. The relay would energise only whilste the engine is running. The feed for the accessories could then be switched via the relay. You would need to experimnent with the type of relay as you really need one that will handle voltages up to abot 20 volts.
I had something similar rigged up to operate my horn in one of my earlier attempts to get a working DC system.
