This solution isn't too recommened. Some drawbacks and problems are:
- Joystick Select is an open-collector output, so HIGH levels may be unstable (this could be fixed by an additional pull-up resistor).
- Bit7 may tend to be HIGH by default, so printing normal text won't work with normal software that doesn't support the mod (the schematic includes a switch to disable the mod manually).
The circuit can be connected internally to the mainboard, or externally to the joystick/printer port connectors.