Michael, This is a very interesting project you're working on.
I just want to add my $.02 to make it easier, consider biasing
your 485 line, (just put a 2,200 ohm res between X+ and +5 and
another 2,200 between X- and gnd). At the PC transmitter,
only enable the transmitter when sending 0's. Put a small cap
(say 100pF) to offer a bit of hold time to increase slew rate when
a logic 1 needs to be sent. I have seen this done many times,
simplifies the Direction Control logic. PS. It works.

Peter, A question for you, I can understand how you could control
the 9th bit out just by selecting the Parity based upon the actual
byte being sent... but how do you recieve that 9th bit by only using
the Parity Error Flag? What if you are switching the parity as a byte
is being recieved... OOPS.. nevermind, I just answered my own question.
RS485 is a half duplex scheme.... Never recieveing and sending at same
time.
But, do you agree that it could not be done in a full duplex situation?
or am I missing a little tid-bit of info regaring the 82x50 series uart.

:)

Josh.