Random message at user’s machine several times per day?

I want to be able to deliver a random small message to our users several times a day. Right now I am thinking it would be great to be able to have a random message from a pool of possible messages to be displayed on the lock screen, a focal point of the display to users several times per day.
1. How would you do this? or
2. What other delivery method would you recommend? (I want to avoid annoying users with an email)

Thanks in advance!