well, you'd need to extend the player controller and add an exec function to it to add a command like "tell' or something, and in that function, find a player by the name given, then call their SetProgressMessage() function with the message you wanted to send them.
Then, extend BladeIN to make your own gametype, and set your new playercontroller as the default.
One major pitfall that I can foresee with that, is that it may be anywhere from problematic to impossible to use that with player names that have special characters or spaces in them.
A "better" way would be to create new menu, which you could open with a new command, which would allow you to pick a player from a drop down list, and input a message to them, passing the player name and message to send as two different parameters to one replicated function, then the server copy of that function would take the player name passed to it, find the player, and then call their SetProgressMessage with the desired message