I doubt there's a script that's out there, but with a little help from the
scripting documentation I'm sure it's certainly possible.
You could have a look at how answering machine plus takes messages and checks them for the key words, then write in a simple function that is set in motion when a key word is found...
As for the function to send messages, you could just have an array of messages to send, and use for...in to loop through the array, using a delay between each message. Something like this:
Javascript code:
//check for message sent, check for keyword, if found send_messages()
function send_messages(){
var message_array = ["hey","how are you?"];
for(var message in message_array){
setTimeout("SendMessage(message)",1250);
}
}
PS: If you really don't feel up for it, I might try making the script next week after my exams have finished 
/ 
