Sorry to bother you again, but I still don't really get it. Are you saying that I can *physically* connect only either a serial device or lights on/cellphone sensor lines?
My understanding now is: if 'lights on' is connected, and a serial device is trying to drive that line to the opposite voltage of what is fed by the 'lights on' function, the serial device will actually drive against 0 ohm impedance. Serial devices with unprotected output will get toasted by this - and you may be in for trouble...
Talking about the other line - if I connect the cellphone sensor line, and a serial device drives this line low, all my other units will go mute. The empeg may know the difference between an actual cellphone call and a serial transmission, but certainly my head unit display and my tuner/amplifier will not, and will start stuttering to the beat of the serial transmission's handshake...

The second scenario will not cause any permanent damage and probably affect only very few people (those with more than one unit that has a cellphone sensor line), but the first looks really dangerous to me! So unless I'm totally wrong (but I can't see where) you should print a warning somewhere saying that connecting both a serial device and the lights on sensor may damage the serial device. To avoid this you could solder a resistor (1k or something like that) into the lights on line before it actually merges with the serial line, so that the serial device will not try to drive against 0 ohm.

Daniel