Is it possible to reply to an opponent's mesage after I've won the game?

Not on the game board, but it's usual to send a PM if you want to say something at that point.

Once the game is over I don't think any more replies within the game are possible. Click your opponent's screen name and select (left side of screen) "send message".

EDIT:

Aaaarrrgggggg!!!! I thought I was going to finally beat Kewpie to an answer in the Help forum.

If I had a life you'd get more of an opportunity. Sorry.

Thanks both