Update 2013-11-05

Update 2013-11-05: After talking about designs with people I've realized that this would be a pretty ambitious hardware project, and would be a lot of work for something I'm not yet sure I would like. So instead I bought a used Yamaha DTX500 drum trigger module ($165 on ebay) which takes weak electrical signals as inputs and uses those to trigger drum sounds. It's designed to be used as the core of an electronic drum kit, but should work with simple p...