Hi,
Andy, what is about another idea called "ring-communication"? All devices are connected 1st_TX->2nd_RX, 2nd_TX->3rd_RX etc. Each slave device re-transmits incoming messages to next device and may insert own messages into the ring; master ejects all the received messages. So, such scheme allows communicate multiple devices via RS-232. Ofcoz, it needs additional software and hardware work; especially it should implement hardware signals passthrough when a device is powered off (for example, via relay).
so, it is still "not fully correct" (=
Good days!