I saw one of these:
http://cgi.ebay.com/ebaymotors/ADJST...Q5fAccessories
if i had the $ to throw around and still was running an int wg, i would try this.
take the signal from the factory wg solenoid, hook that up to a sens wire on a relay, then push full 12+ from batt to this solenoid to open/close the int wg.
not sure if this throws out, or pulls in on charge, but either way could be modified with a cable/lever system.
just a random thought