Not quite what you're looking for i know, but I've posted incase anyway...
If I remember correctly you can connect a PC serial mouse to the back of your ST (the serial port there) and use a driver such as Gem-mouse (think thats what it was called). However, this won't be any good for games etc.
If you're good at electronics (Im OK and to be honest, these aren't that difficult to make) you can build a PC to ST adaptor for a serial mouse. The schematics can be found at
You will need to program the PIC controller however, which may cause a problem for some people.
You might find stuff here too:
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