hmm.. I've just realised that 3 bits would just enough for the message (in both directions) so I should be able to use 4 pins on each device, 3 for the message and 1 to say whether sending or receiving.
The sensor board would check the state of the control channel to see if the main board was sending a mode change, and read it if necessary. Otherwise the sensor board will output the message.
Looks like I wanted parallel comms not serial !