You can send a mass message. Copy your message and paste it in a message box for each player.
I frequently play in my own alliance so maybe my opinion does not matter. But no I would not like it.
If I was in an alliance with 20 people I would be likely to stop reading messages. Maybe I would just block everyone except the leader and first officer and tell the leader to copy anything important. Then I would use the mass message to tell everyone they are message blocked.
Maybe allow mass messaging inside of your general group. But then copy and paste is not so hard.
Maybe if mass messages were separated from other types of messages. So personal messages would be yellow and mass messages might be pink. If I could ignore the pink envelope and never had to read or delete the mass messages then they would not be a problem.