I do not know if you're experienced with the trigger editor, but the only way I can see that happening is to make a custom chat box with game messages and filter who does and doesn't get the message.
Also, as defskull stated, by forcing players to talk to the obs channel, you effectively remove the ability of players to speak to each others (except if there's observers).
Result, normal chat is hidden to everyone and custom messages are displayed to the specified players in the triggers.
Voilà!