You could have those files (wiki, todo and so forth) in their dedicated Git repository, and then add them to a development repository through the subtree merge strategy (as described in this question).
That way, they get included in any development, and they also get updated by those sub-projects.
Any modification in a branch doen by a sub-project can then be merged back in a "main" branch in the original "wiki" Git repository, and all sub-projects can merge their own development branch with that main "wiki" branch in order to update this content included through a subtree merge.
Not exactly an "orthogonal" versionning, but still a practical solution.
