As far as I know mini and micro usb are electrically compatible, so you could splice together your own cable on the cheap if you have two connectors you don't mind destroying. No circuitry should be involved, just direct wiring. Correspondingly, the adapters should be pretty cheap anyway.
Android 3.0+ (and maybe earlier) will support mounting usb filesystems, but I doubt anyone expected the micro usb port to be used for this. I imagine some additional software will be needed. You'll have to root the device, as well.