How to convert from 2 data line full duplex to 1 data line half duplex?
There is a protocol which is sensibly called "1-Wire" in which only a single wire is required.  This protocol requires a master to provide collision avoidance.  I don't know of any way around this - you must either have a single master or you must allow collisions and do collision detection.

Anyway, it is supported by a number of devices through firmware and possibly still some hardware.  To replace a full duplex design, the master would have to set the rules, prompting the slave for data, and the slave would have to wait until the master allowed the transmission, but I think it could be implemented easily with the inevitable loss of data rate.

Good luck!