I guess this -could- be done with standard hardware on one line, with proper software and protocol.
Say, there's one "master" device, and every device on the line is assigned an ID. The master sends a packet of data that contains its own ID and the ID of the target and all the devices except of the targetted one stay quiet - only that one is allowed to reply. Of course all the other devices "hear" the reply, but since it contains only the master ID, they ignore it too - only replying to messages from the master addressed especially to them.
Another method could be a "virtual token ring" - one device is allowed to "speak", all the others listen. Then the device passes the right to speak (a specific message) pointing to the next device in the ring, and goes quiet until the token returns to it.
No normal continuous, two-direction RS232 communication would work though, and besides RX and TX would be the same both attached to the same line and I'm not sure how the hardware would handle that.