I have a
thread on a feature similar to this. I could easily do it for you when I get the chance, although I'm not sure about an animated one. I'm just about to go out so I can try when I get back if you need any help, although the main part of the coding is in
the 5th post.
Hope it helps.
Edit: Sorry I just realised your actual issue is probably detecting when someone is typing? I'm not sure if there's a script function in Plus! to do this or not... If there is, my thread should help. If not, I'm not sure. Sorry.
/ 
