How about fitting another solenoid at the battery with the actuating and thick brown wires slaved to the original ... it would mean the long unprotected run would only be live during starting and fusing the ancilliary feed at the battery end would be much less problematic ... probably a lot of faff when, as RichardV6 says, it's a rare thing to have a problem with .... probably easier to have a motorsport style cutout in the earth side as a failsafe if peace of mind were needed.

K