Thanks for that, but is the "BMW Auto OFF Power Socket" thing you mention the accessory outlet on the bike? I'm going to have to move some stuff at the back of my garage and pull the tarp off my K1300GT to check, but I'm pretty sure that on that bike at least, the accessory outlet DOES maintain power for about the first 30 seconds exactly as the GPS connection does. Could it behave differently on different model BMWs? If it does shut off immediately with the ignition on the F800GS, then (if you can get to it) it would be a great option to use as the trigger for a relay.
If it does shut off immediately, then why the heck hasn't anyone else figured this out? :)