Can you safely "short circuit" 2 signals when they compete for the same pin to avoid having to use a multiplexer?
I'm having a hard time understanding what you're trying to do, but as long as you think about all the various states the components could be in and put the appropriate current limiting resistors on, I don't see why not. Beware that inline resistors can mess with high speed communication, I don't know how fast your RS-232 is running.