I could see a system being both easy, and hard to implement; Without thinking about it too much, it'd require a hash table filled with every single already-used key that is always perfectly synced with every other instance regardless of each instance's chosen federated instances.
Best bet might be to just have a non-linear, randomly chosen, hash with a fixed key-length backed by a very large keyspace. This could effectively defeat the need for perfect syncing due to a greatly decreased chance of collision over short periods of time (statistically covering any minute-or-two long desyncs).