Interesting article, and coincidentally relevant to a project of mine. I won't spam you with a link, but here is the approach I am considering doing, I would appreciate comments:
1. My own code in this project is AGPL.
2. The project bundles some other pre-existing libraries, with compatible licenses, like zlib, Apache, etc.
3. For contributions, I let people either (a) submit it under a compatible license like Apache, in which case they hold sole copyright, or (b) submit it as AGPL, with joint copyright assignment, allowing me to relicense it.
In other words, I am already bundling code with compatible licenses in this project, and people submitting new code under a compatible license is basically like more such code. Or, if they aren't comfortable with a permissive license like Apache or zlib, they can use the AGPL, but then I guess they need to trust me regarding other licenses I use it for.
Does this seem like a fair arrangement? Also, is any project already doing something like this (I can't seem to think of one)? I hope there isn't some fatal flaw I am missing.