Last night I woke up thinking of a second redundant hydyraulic system for the rear brakes only. Perhaps a T-1 master, or some small slave cylinder attatched to a handle that could apply force for braking....Then I says to myself "self you could plumb this into the existing line going to the rear of the car and put a one way valve in that line to keep it pumping the rears only" Also a one way valve would be need to protect the e-brake pump from the main master cylinder. I know a cable is the ultimate redundant system, but this could be a cool compromise keeping complexity/weight/junk off those rear wheels. or :