I had a look at https://fedoraproject.org/wiki/Packaging:Conflicts today and noticed that it has a section on "Optional Functionality" which sort of describes how you deal with something relating to weak dependencies back before we had weak dependencies.

I'm thinking that at least some of the stuff there is wrong now, and that there must now be a way to do this without a Conflicts:. Except that I don't really know enough about all of the weak dep interactions to say what should happen, and so I can't submit a draft. I'd be happy to make one if someone gives me a couple of hints, though.