(note, my experience at the hardware level is limited, so just ignore this if it's completly off)

Couldn't you build the multiplexor to support flow control? If one device is transmitting or recieving, toggle the flow control on the other devices to have them wait until the line to the empeg is open. This should work with more devices compaired to trying to deal with custom programming on every serial device that might sit on the multiplexor.