Well, any of the *.sh or *.shm files that accidentally shipped with games would be useful (like the one you already provided). I hadn't thought much about how to let the user edit messages, but I may as well use the same system Sierra apparently did. e.g. both the editor and the script will pull the message-related defines from the *shm header files, and the editor will also push new values into there. And then I guess I'll have a global talkers.sh that is included by the game.sh, and edited manually by the user.
What does "MPC" refer to?