In melee if I use Greatjon Umber's ability to join a military challenge in which I am neither attacker or defender, am I considered to have won (or lost) a challenge?

I'm just asking for purposes like Robb Stark (Core)'s ability and stuff.

EDIT: I know someone will say 'who cares' about melee, but even in joust it can be relevant. If you do count as a winner or loser, you could have him join your opponent's side while you defend a military challenge to guarantee a trigger of Siege of Winterfell and get those last two power. Based on this abuse I think it is probably not going to work, but it would be interesting.

You can't win or lose a challenge unless you, the player, are an attacker or defender. Making Greatjon Umber participate in another players' challenge does not make you an attacker or defender. So you would not be able to trigger any "After you win/lose..." effects.

Thats what I had thought, but I wanted to make sure. There was potential for hilarity

There still is. You are not a winning/losing player, but Greatjon is a participating character on the winning/losing side. So effects that target "participating characters" without specifying control by a winning/losing player (like, oh, Deadly, Pit Viper, Foamdrinker, etc...) could hit Greatjon even though you did not actually win or lose the challenge.

Yes, that I know. And deep down I knew it wouldn't work. But using Greatjon to jump into a challenge to have Robb Stark (core) trigger. It would have been very fun.