Right. So as I thought, a bunch of random contributors can't be considered to be co-authors of a "joint work", which for all I know is a concept peculiar to certain jurisdictions. Indeed, merely accepting contributions which are appropriately licensed might well be a strong indication that each contributor regulates their own work and how it is used, severely undermining any claims that everyone is acting as a single entity whose intentions can be subverted by paying off one or more contributors.
I still think contributions through the normal practice of distributing code under a project-compatible licence is the way to go: that way, a project and its contributors have equal standing, and no extra magic is required. Indeed, as others have pointed out, there needs to be some convincing justification for having that extra magic around.
That said, a set of bumper stickers to communicate that magic (or lack thereof) would be a helpful thing, and maybe that could be the principal benefit of the initiative in question.