If they really wanted to get you, they'll look underneath, and see wires/brakes only on one axle.
Realistically, I've never seen a cop do this.
Now, if you got into a accident because someone stops short in front of you and you can't stop in time(which is certainly a realistic scenario)... maybe then they'll look. I suppose if your insurance company were bastards, they could deny the claim or something...
never heard of anyone having trouble due to this, and there are thousands of them out there with brakes on just one axle..