Sounds like you are wanting a lightweight networking protocol, using the physical layer as an ether (in the original sense of ethernet!). So some sort of collision-detection or retry mechanism is needed. Just make sure the physical layer can handle collisions without damage.
You might want to look at some of the simple packet schemes used in RF transceiver modules such as the nRF24L01+ - you then have a framework that could be easily switched from wired to wireless. There are no doubt a handful of relevant open-source networking protocols to look at that could be suitable. Have a look through the forum archives esp. the networking/protocol/devices forum I guess.