It would be interesting to see someone spin up an instance, no signups, just Lemmy-Federate. Check how much resources it uses up.
But for an instance that already has a lot of users it's going to be a drop in the bucket. Any large communities probably already have a real user following them, and any small communities won't have enough activity to cause significant load anyways.
You could do it as a DHT network (distributed hash table)
https://en.m.wikipedia.org/wiki/Distributed_hash_table