HTC might not condone macros, but you could use Voice Attack. It's made for acting on voice commands, like if you say "Engage warp drive" in a space game, it could retract weapons if they are deployed and then activate the warp engines - all by passing the corresponding key presses to the game. Along with voice recognition, you can assign a keyboard shortcut or a controller button press.
It would be easy to create a macro that would open the text radio to the channel you want, then enter in a preset message, then (for safety) do nothing else; you would manually hit enter to send the message.
