I think nobody will ever *consider* starting working at this, but however I'll tell you.
I think it's impossible having emoticons being replaced as you type right in the textbox of your chat window; anyway I suppose it would be possible having a second panel on top (or bottom) of the message field, which would show how your message will appear once sent (complete of emoticons), updated as you type (or on idle, as you stop typing).
I think it is theorethically possible, but I don't have time nor I am into tcl/tk (and aMSN) enough to even *think* about implementing that. I've just tried to see how DualDisplayPicture hooked the window and I decided to forget my idea
so I'm just telling you; maybe some nice person will want to try