4D6,
I haven't done this but I have a few thoughts on it. A 6 volt starter can pull 200 amps when it's really working hard. The other concern would be that the relay, if it's electrically held in, would have to have a duty cycle that allows you to drive for extended periods of time with it energized.
Your method of energizing the relay would have to be wired directly from the battery or the battery side of the relay for it to operate, it would be powered all the time.
Have you thought about one of those twist type battery disconnect switches mounted somewhere convenient?
I think this has been discussed here before and the problem was finding a solenoid (relay) that could handle being energized for hours at a time, as you drove around. Most were made for momentary service. There were some for maintained service (I think) but they were quite expensive. Good luck.
Stuart
