There's an AutoTalker script which lets you easily activate a "talker". Talkers can be a .txt file containing words to replace and their replacements or they can be a .js file with a main function to transform a message. StuffPlug 2 also had an AutoTalker function, and there even was an emotitext talker written for it, so it won't be a hard task to rewrite it to AutoTalker. But if that script doesn't give you the needed requirements, you can always make a separate script of it.
OFFTOPIC: And yet again we bumped a thread!
Ah well, it's better than pointing people to this old topic when they create a new one. It saves tid's.