This is a simple mod that adds a new block - The Proximity Messager

This new block, when interacted with in creative mode, opens up a GUI where you can set the message to be displayed. You can also set the activation distance and a cooldown in seconds (if you don't want re-entry to the area to trigger the message again for a while).

Every 2 seconds, the block will check for nearby players in the confgured radius, and send a message to all of them. The message will be sent in General chat, or the Proximity chat if you have The Basics mod installed.