OK, I've solved the problem. I bought a reversing module and connected its output to 'A' and its input to the the DCC BUS. Isolated joints as shown by the blue dots. It works fine as long as the reversing section is as long as the length of a full train. Initially I had the reversing section too small and I had some problems with some Hornby coaches shorting one one side of the set-up when the loco was on the other.
I actually bought two quick response solid-state reversing modules for about $40 each from Tony's Train Exchange
. Click here
for more info. The second I'll use for a turntable. I mounted them in a clear VHS video box that is mounted under the layout. This way, when the box is closed, I can see the LEDs that indicate if power is being received and also the LED that indicates the polarity of the system - a useful debugging tool.
BTW, Tony's Train Exchange
sell Lenz decoders for less that you can get them in Europe (including transport back to Europe)... go figure