Embedded above is [egyokeo]’s solution for using MIDI drums with Guitar Hero. He’s playing a DrumKAT MIDI kit. It connects to a PC running his MIDI Hero software, which handles timing and multinote combinations. The PC uses a USB ToolStick microcontroller to send commands to the console using an FPS adapter or soldered into the instrument. It’s a fairly good solution if you’re building a generic controller and need to modify the signaling.
When Rock Band was first released, modders sought to adapt their MIDI drum kits for use with the game. The easiest solution they found was Highly Liquid’s MSA-P. It’s a photorelay based MIDI decoder. You need to solder directly to the brain in the Rock Band drums. If you’re planning on modding any instrument, check the compatibility matrix first. Hopefully you’ll end up with something that can be used across multiple games.