I saw the breakaway accidentally activated on a 5'er, while still attached. It didn't lock the wheels, it just put a terrific drag on things, the driver instantly knew something had gone wrong and pulled over in a controlled stop. I'm pretty sure the breakaway won't lock up the brakes on my jeep with a brake buddy, either; it's an on-off system, and it doesn't lock up the brakes when it activates normally, so it isn't going to do it when the breakaway is activated, just bring things to a halt. All the breakaway does is tell it to hit the brakes, it's only got so much air pressure. Since I can feel when the toad brakes activate normally, I think I'd notice if they activated abnormally (plus, the little red telltale would light up). Of course, the more sophisticated systems might work differently, it'd be interesting to know how aggressively they work in a breakaway situation.
I agree that the first line of defense is to do the best possible job of connecting the tow-er and tow-ee, but I still plug in the breakaway as a just-in-case. I sure don't want to kill or injure anybody.
2003 Fleetwood Southwind
Still employed, sort of, and counting down........