"I think it weakens the argument you are making to include them as examples, unless you can point to some sort of previous discussion in the context of upstart development where good-faith contributions from contributors were turned aside by the assignment policy requirement. I'm not aware of that ever happening."
Technical design might be the more important reason for systemd but it is clear that copyright assignment requirements played a role and while what you have suggested is one way to look for the impact, I think it is far from the only way. For instance, the questions one can ask include, how many developers were turned away from ever submitting patches because of the requirement? Among the active systemd developers, which ones felt more motivated to systemd because it explicitly advertised the fact that no copyright assigned is required? Did it influence adoption by distributions? and so on. I don't purport to know the answers to these questions but I disagree that it is not a valid example although it might not be as big a instance as Openoffice.org vs Libreoffice.