The guild achievement messages aren't used for the announcement you are looking for. I've opened a new feature request for you, but it's not clear as of yet how difficult it will be to implement. It's also, by definition, difficult to test.
There are no plans to support chat between non-connected realms.
Two of the design requirements for GreenWall were to 1) handle extremely large guilds and 2) require minimal configuration (either explicit or implicit) by the typical guild member. Using BNet chat for non-connected realms would defeat both of these requirements, it wouldn't scale to handle large guilds and it would require a significant amount of configuration management for every user.