Method 1 (easy)
Download ModernRcon 0.2 after joining up on www.codhq.com's forums. This version was just released yesterday. It is specifically for COD4, and now has auto messaging abilty. The downside is you have to keep the tool up and running for the messages to scroll.
Method 2 (a little more involved, but not dependent on client side Rcon tools to run)
PB_SV_Task [X] [Y] [command]
Adds a task to PB's Task List; The Task will be executed X seconds after entry and every Y seconds thereafter; use -1 for Y if a one-time task is desired; this can be used to execute game server commands as well as PB commands
Create a file called (just an example, you can call it anything you want) svrmsgs.cfg. Add this text to it and edit for your needs using the explanation above from evenbalance.com:
// Task setup for PunkBuster
//where <color> =
//^1 for Red
//^2 for Green
//^3 for Yellow
//^4 for Blue
//^5 for Cyan
//^6 for pink/Magenta
//^7 for White
//^8 for Black
//where <message> = the text you want to scroll
pb_sv_task 10 1000 "say <color><message1>"
pb_sv_task 100 1100 "say <color><message2>"
pb_sv_task 200 1200 "say <color><message3>"
pb_sv_task 300 1300 "say <color><message4>"
pb_sv_task 400 1400 "say <color><message5>"
Then upload the svrmsgs.cfg file to your pb directory on the server, and edit your pbsv.cfg file to call the svrmsgs.com by adding "pb_sv_load svrmsgs.cfg"
Finally, do a pb_sv_restart and you should see your messages start to scroll after the first X value of seconds has passed.