Hi, Mark:

I'd agree with Adrian here. There are just too many alternatives to have a complete database. It's like tablature programs that never seem to come up with useable tabs from a guitar transcription.

Basic chords are more or less "public domain", so, other than the convenience of having the database readily available, I don't know that you'd have to license something.

I came across a need for just this feature this week, actually.

David