there are a lot of players who don't participate in forum discussion so is it reasonable to expect them to post here about the challenge? or is the whole point in requiring them to post here an attempt to get the engaged in the forums?
if the ladder is not the case, would a screen shot of the agreement be sufficient?
An SS probably shouldn't be accepted just because its very easy to take someones words out of context in the chat room.
It would be very difficult to keep track who is challenging who, who won, etc. We have players post there challenges here and it keeps everything much more orderly. It also gives streamers like Xurnt and Mousey time to sit in on the challenge. Remember simply beating a master tag holder or a challenger without them agreeing to the challenge isn't accepted.
If players decide they don't want to be part of the forums that's their choice. However without joining the forums and posting in this thread they will not be able to compete in the master tag challenge.