The Serial data goes thru the Paula chip, and the contol lines come from one of the 8520s. If you've got a spare Paula, swap it out & see what happens.. You can swap the 8520's since there are 2 in the machine. However, the serial port is buffered through a set of driver/reciever chips which are soldered in. Incidentally, this is one place where the 12v is used.
There ought to be a way to do a "loopback" test by jumpering transmit data (pin2) to recieve data (pin 3) and using a terminal program in half duplex... Off hand, I don't remember if any of the other pins need to be connected for this..
