To be honest, I've never even thought about this.
It's a cool idea, though. And a great exercise for a weekend project. 😎
It should be possible to use something like an Arduino Leonardo for that purpose.
If I'm not mistaken, that model has a true USB controller to emulate HID devices.
Maybe it could be reprogrammed to operate in reverse, converting a HID device, like an USB joystick
to an anlogue game pad.
It would essentially provide signals for the digital buttons and two analogue axis unsing the PWM/analogue pins.
This would also require a bit of external electronic on a shield, like some relays/optoinsulators for emulating the
digital gameport buttons and some resistors for the axis.
If only a simple gamepad is to be emulated, a simple d-pad (four directions) could be simulated by
using some simple resistors with fixed values. These would have to be "turned on" by using transistors/a shift register.
A more elegant idea would be to use some special hi-fi ICs for volume control, but I don't know much about them.
Anyway, that's just an idea.
Converting USB to Gameport shouldn't be hard with todays microcontrollers,
you'll only need the "know-how" to do it. 😉
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