Perhaps look into software from this company:
http://supermegaultragroovy.com/products/capo/

The Capo app is pretty danged awesome. Chord detection, slow downer, pitch modifier, etc. I've only witnessed the iOS version with my son's guitar teacher.

I see you would like open source. This one is 10$ for iOS. For what it does, it's an absolute steal.

Joe V, this could be your voicing solution as well.