I think you can already do this with spawn prisons; just let only skinned players in. Automating it won't change whether or not it can be done. Besides, it's his server; he can let whoever he pleases in. Don't tell him how to run it - go make your own if you want to run things your way.
If this were implemented, a server label would need to be integral. Also, this wouldn't stop griefers.
This would inspire alot of hate against the 'Free-ware' as sandbox will always remain free, and should remain like this. If you got a problem with people joining, or trust issues, do your own thing or set it to private.
Anyone who disagrees with having paid-only public servers is free to make their own open-to-all server. You don't have to go to a premium-only server. If no one can agree on anything else, let them at least acknowledge that a server owner has the right to run his server however he pleases. If someone wants a public premium-only, more power to them.
I thought it was a good idea but everyone has to fight over it so now it's a sucky idea =b
I wanted premium only because I like having skilled players join my server (of course I assume those who buy the game like it enough and have plenty of experience to be skilled..)
What I don't want joining my server is munchers, griefers, and noobs who have not bought the game yet.
And I don't want to sit there with an unbreakable cube governing who gets out and who doesn't.