I see what you mean what a clever way to do this =)
But i still dont get it how the cab would be able to differentiate between two trailers
as with your earlier explaination
Hi Zoom

It would work if i get what DT is saying is that if you can turn the normally ON solid state relay ON/OFF (with serial Data say 01010101 ) fast and quick enough for it not to be noticed by the naked eye,this data can then be read by the pic in the Cab from the hall effect current sensors analog output fed via say a Schmitt trigger you should be able to recover that 01010101 Data sent by the trailer
This way each trailer would have a different code differentiated by the Pic in the Cab

Hope i am right
Isaac