Use of the GPL doesn't merely prevent commercial developers from creating non-compatible formats -- it prevents them from using Concord at all. Let's say I want

Use of the GPL doesn't merely prevent commercial developers from creating non-compatible formats -- it prevents them from using Concord at all. Let's say I want to build a commercial app that happens to use Concord, and I'm happy to use Concord as is (contributing back any patches or improvements I make) and even make the outlines exportable (assume outlining is just one feature in a much larger app, not the main focus). In that case, I'd have to make much or possibly all of the client-side code GPL compatible, so I can't use Concord, even though I'm not violating your rule about compatibility and am contributing back to the Concord community. GPL seems like overkill given your goal. It will be limited to a niche of either fully open source public applications or completely private/internal applications.