sealg,
First off, you can get a break-away system for much less than $200 to $400 like you just mentioned. Readybrake's Readystop breakaway system, which will work with just about ANY vehicle or setup, is around $100.
I must say, though, your solution does sound interesting, and you've definitely done your homework with this very well. Kudos to you for that. My only concern with this, that may or may not be a concern, depending on how your Jeep truly is designed:
Break-away systems are designed to brake ALL FOUR wheels via the regular brakes. Not just the rear brakes like an emergency brake does. Engaging just the rear brakes at highway speed, in an unmanned vehicle....Well, that just seems like even MORE of a recipe for a huge disaster, even more so than if all 4 wheels were being braked. Much higher chances of it spinning and going who knows where, I would think.
Mowermech does make a good point, that the results of a break-away incident are not going to be pretty regardless, but I'd still want all 4 wheels to be braked in this case.
I know you said that your Jeep's emergency brake engages all 4 disc brakes. If that is true, then your solution should be OK (as long as you wire it right).
However, I would make absolutely 100% sure that it works that way (braking all 4 wheels) before going forward with this. Some comments previously about emergency brakes supposedly only braking the back wheels, makes me wonder if your Jeep's emergency brake does truly brake all 4 wheels.
