moved this question to this forum, since I no longer use the older forums.
You could use the api to change your auto-response. In this approach you would need to write a program that you would run once a day to perform the change.
Alternatively I could write an app for you that would allow you to enter the messages and automatically rotate them on a given schedule. I have already done something similar for a smoke cessation program which has a phone number which users send an sms to when they lapse, and the system sends them an inspirational message in reply (there are 30 odd messages and those are rotated for each individual user). If you are interested send me an email with more details and I will work out a quote.