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I am actually working on this very thing..
originally it was just a side project, but it has taken on more urgency since my GS-280 controller seems to have failed completely sometime.
A few things: (some might be obvious, no offence meant if they are!)
12v into the stepper motor casing. From there it is not actually connected directly to the stepper, but goes out the mini-DIN (PS2 type) cable into the GS-280 controller. The rest of the wires are used for the handset to drive the stepper, using a ULN2003 chip (controlled by a microcontroller) to drive the stepper - this uses the majority of the rest of the wires in the PS2 cable. None of this really matters of course.
Creating a PC interface to the controller should not be especially difficult, however is of limited use.
The only thing you can control is forwards and reverse speed, so the only reason i can see to do this is for extended length drift alignment using a relativly wide field of view - ie, DSLR + standard lense, using the "V" alignment method. That was one of my reasons for wanting to do this. The hardware required could all be picked up at Jaycar for quite cheaply - Arduino (or compatible), ULN2003 chip, and however many extras you want to add (sockets, perfboard, USB bulkhead mounts, buttons, etc)
I have actually added a second stepper to my EQ3, so i am working on something that can be driven with LX200 commands - I recently found a similar package, Astroduino, which has done a lot of the hard work, which allows LX200 commands to be used to drive an ST4 style autoguided system.
One unfortunate thing i have found with me EQ3 is it seems to have a quite severe amount of noise in the gears - even after a teardown, clean and regrease. I recently ttried using some rogue and an electric drill to "smooth" them down a bit, but it was when i tried to test this i found that my GS-280 controller seemed to have failed.
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