cubsrule17,
Not exactly what you want but this might work for you. Check out a program called PhoneAlarm from PocketMax Software:
http://www.pocketmax.net/phoneAlarm.html
That allows you to create profiles that effectively change the phone configuration at specified times. One of the profile options is to start/stop a specific program at the designated time. So, for example you should be able to create a profile to start the messaging application at 7:30am and another profile to stop it at 5:30pm.
I'm not sure how to launch the messaging application, but you might be able to find that around here somewhere. Now that you got me thinking about this I have to try it myself - I'll post the answer if I figure it out.