Could you create an adapter card that was a subset of your other 8052's and this accepted normal RS232 data and then set bit 9 when it retransmitted it to the bus? This adapter card could then check bit 9 on any other messages from the bus and see if this was addressed to the PC, if it was it could send it on to the PC via RS232 or else just dump the message.

Just an idea - I have not actually tried it, but that is what I would do, as you could then program the PC really simply (even use VB)

Marshall