I don't see why not. I'm almost positive this could be done using mIRC's socket support. In order to write a script to do this, the on KEYDOWN event would need to be triggered in chat windows rather than only custom windows.
One thing about using a script, both users would need the script installed.
* An idea on how my script mechanism would function:
When starting a chat with another user, the script would set a socket to listen on your ip and on port 2112. When the on KEYDOWN event is triggered by you, the script would open a socket to the other user's ip and port 2112. Once the socket is connected, the script will have the socket send the string 'TYPE' to inform the other user's client that you're typing a message.
The other user's script will recieve the 'TYPE' string, and from here, alert the user that you're typing a message. This would repeat each time the on KEYDOWN event is triggered in a chat window.
The point I'm trying to make is, if this can be accomplished using sockets, then I would imagine Khaled could implement this very, very easily using a method like the one I suggested.