Shoutbox

This is possible and you can achieve what you want using a program called Messenger Countdown. The purpose of this program is actually to make a countdown to certain date, but you can type whatever you want in the text box for it to display.

I can't find the website for the application, so i've just attached the file.



As you see from the picture, there is a date to set a countdown to, but if you just want to type in some text, leave the date and type your text in both boxes where it says text to display.
Then choose your icon from the 3 at the top left.
If you're not doing a countdown, and just want some text in there, you can set the refresh rate to the highest value (120s)
Then press the button with the blue arrows and the text will appear on your personal message. (Note: You will have to wait the ammount of time in the refresh rate before the text first appears. So, at 120s, you will have to wait 120s after you press the blue arrows.

Result: