Hypothetically, it be possible to achieve the same thing with a shared tailscale network, right? But can you kinda "federate" tailnets like this? So independent tailnets owned and run by individuals, but networked together into an inter-tailnet?

You miss the fun and games of running your own DNS infra etc I guess.

This is one of those cases where the work is the fun. It just depends on your personal definition of ‘work’ and ‘fun’.