SASI will be a bit more complicated since we don't have schematics (there is some small custom DMA circuit that needs to be emulated and isn't described in the manuals).
For now, I've added the two RS-232 ports to the driver. Here's how to test them:
Start MAME with:
mame psi98 -flop1 kos605d -flop2 util605d -rs232a null_modem -bitb socket.127.0.0.1:1234
(attaches the null modem to serial port a, and connects it to port 1234 on the local computer)
Start PuTTY now and tell it to connect to localhost:1234 with connection type "raw".
Let the emulated system boot now, open the TAB menu and configure the RS-232 port to 2 stop bits.
Then enter the following commands:
(load serial port a driver)
(attach this driver to input channel 5)
(attach this driver to output channel 5)
(start simple terminal program)
At this point you should be able to see any entered keys in the PuTTY window, as well see everything typed into PuTTY in the emulated system. Here are some screenshots:
Pasting ASCII art: