This is not that big of a problem. A third party can always make a bootleg version of metacommunity post viewer. Lemmy can already view posts across all federated instances using “All” tab, so all that would be needed is some method of meta labelling of same (not similar) communities across instances and to be able to view unified communities under such tags, and this tagging be done appropriately by all instances’ admins in synchronisation, ignoring their personal and ideological differences.