http://puu.sh/JNThis sort of answers your question. Okay the photo doesn't actually speak for itself, so I will: that's my taiko rig. The drums are a set of Roland V-Drums with a TD-03 module and four PD-8 pads. Using a standard 5 dollar MIDI cable, I've connected the Rolands to an old GHWT drum kit (which you can't see in this picture, just trust that it's there) and mapped the MIDI signals to the GHWT joystick buttons (I think 3 of the MIDI codes GHWT uses are 30, 38 and 48). You don't need to use a GHWT drum set; any midi adapter will work. I just happened to have it.
Anyhow, the drums send codes to the GHWT kit when the drums are hit, and that sends a signal to my laptop, which is where the GHWT drums are plugged into. Using joy2key, a free and easy-to-use key mapper, I just translate the joypad signals into standard keyboard presses (zxcv). Once you've got that set up, you can pretty much use it to control anything.
But thanks to GHWT using laggy wireless technology, I've had to set my UAO to like -180ms to compensate for the input lag. Using a male-to-male headphone jack, I've sent the laptop sound to my drums' headset, so I can get feedback from my drums while playing the music in osu!. Simply disable the sound effects via the options menu and you're good to go.
I'm pretty sure I made this sound far more complicated than it is.