On Lemmy you can see (and search) a list of all the activity from every instance federated to your home instance. Looking at Ibis, which a few posters have mentioned on this thread, it has a discover page with a list of federated instances and articles on those instances. The current format is hardly scalable, but it's a start.

But, as I said before, the issue is less about discoverability and more about editing. Just like I can post in this thread even though I'm on a different instance, you can edit an article on one instance even though you're on another. The alternative as used by Wikipedia, is to allow anyone, account or not, to edit. Requiring someone to have an account on a federated instance would mitigate a fair amount of spam and ease moderation.