I vaguely remember seeing that.

If I wanted to do that I would use a 1pole double throw relay (spdt) with the reverse lights activating it.

relay common - brake solenoid
n.c. term - power from delay box brake connection
n.o. term - hot 12volt (line side of delay box)

This would isolate the box when the relay is applied and work normally the rest of the time

Read the rulebook though, about transbrake and 2step connections. Some tech guy could have a problem because you are adding a device into the circuit