For a system that uses a serial chip where our implementation subclasses device_serial_interface (currently the 8250/16(5/6)50, Moto 2681/68681, MOS6551, or Intel 8251) it's mostly a matter of adding an RS232_PORT device (which connects you to the terminal/null modem) and doing a little "wiring".
This commit shows the basic wiring and this one adds the actual terminal and null modem. (The null modem can output to a PTY or TCP socket; it really needs bidirectional which I'll add if Carl doesn't first).