You can use RS232 for the commands from the PC to the slaves - because all the slaves need to receive the same commands simultaneously;

But RS232 is not suitable for the responses from the slaves to the PC - because only one slave may transmit at any one time, and the others must effectively disconnect from the bus.
You might be able to achieve this with clever use of the shutdown facility on RS232 transceivers that have that facility...

Depending on the speeds & distances involved, the simplest approach could be to use open-collector drivers in a Wired-AND connection, and then a single MAX232 (or whatever) to connect to the PC COM port.

But even then you still have the issue of how to get the PC to support 9-bit mode...

Perhaps it might be easier to have the PC use a standard RS232 link to a "Master" 8051, and then let that handle all the slaves.
You'd have to devise some protocol between the PC and the "Master"...