"With one of our dear '52 derivate with two uarts to interfacing any computer with rs232 and the others '52."

But the PC needs to be able to "see" the 9th bit, so that it can properly decode the protocol (and, later, set it to be able to simulate a node).

I could add an extra byte to carry the 9th bit, but then the PC would be out of sync with the real bus timing. :-(

Actually, I already have hardware to do that: a Triscend E5 with a half-UART receiver monitoring the "9-bit" RxD, another half-UART receiver monitoring the "9-bit" TxD, and a half-UART transmitter sending "normal" ASCII data to the PC:
when it receives, eg, 0x5A on the "9-bit" link, it sends "5A " (ie, a '5', and 'A', and a space) to the PC;
if the 9th bit were set, it'd send "[5A] "

But, obviously, that means that one byte on the "9-bit" link could take five bytes to send to the PC!!