1) the PC is singularily unsuited as a master controller
2) RS232 is not multidrop, RS485 and 422 are
3) the 9-bit mode is what you need

The way to do it is to have a dual UART uC master that alone communicates with the PC via RS232 and then communicates with the slaves via RS405 (or 422 if you prefer) in 9 bit addressing mode.

Erik

ps "The Bibile" of MCu by Sir Erik Muland,
1) "the bible" is not by me, I only quote it
2) No, Andy, I have not been knighted - might like to, but not possible in the US :)
3) it is Malund, not Muland