Are there any PS serial cable mods out there? So, can you use the serial cable (reterminated with a DB9 connector) and use it for any application on the PC?
Er, yeah, there were. However, it's not a case of just soldering a DB9 (or DB25) connector on the end. The PS serial port operates with voltage levels that are 0V and 3.3V. Typical PC serial ports (RS232) operate at voltage levels that are -12V and +12V. This means you have to build a converter to convert the RS232 voltages to something that's safe for the PS.
Skywalker produced some stuff /ages/ ago for this, and you can find it on the Hitmen Playstation Archive. (http://hitmen.c02.at/hitpsx/hard-link.htm)