@Andrew - Well, maybe the soon to be renamed Gorilla Theme project will fit the bill and perhaps I'll participate in that group and see what happens - https://www.wptavern.com/matt-takes-notice-of-gorilla-project

@Brad - I have asked him this question before. In fact, I'm pretty sure I asked him about child themes as well which are mostly CSS and images. Please see this post and read down until you get to the point where we discuss that aspect of themes and the GPL.

http://www.wpsnippets.com/2008/12/transcript-of-wordpress-weeklys-interview-with-matt-mullenweg-december-2008-part-2/

Even though Matt may know the GPL inside and out, it's hard to say one way or the other because no court cases have gone through to set a precedence on what is and is not GPL compliant.