You have no way to discriminate which alliances would be included or not included. This is a silly idea furthermore because you wouldn't be able to restrict entry to forum alliance members only.
Someone's thought of it, and it didn't happen. Move on.
No - it's more than possible to restrict the access to forum alliance members. There would be a password to use the world, and the alliance leaders would be given the password, which would change each round to prevent leaks. Then, the alliance members are the only ones who can get in.