You are
going to need to prevent non-transmitting inputs from frying
themselves probably by tristating any input on a machine that doesn't
have the token. Even if you have them tristated you are going to want
current limiting protection so you don't blow out the ports when a
problem happens. I doubt that the C64 serial ports are specified to
survive having multiple transmitters on the same line.
Brain not working this morning. I obviously mean outputs in the
paragraph above.
Eric