Hmmm...
I'm kind of bad at those questions, since I most always GM 4e, and thus I don't spend a lot of time pouring over which items I want, but I'm sure there are some. I mean there are definitely some powers that let you grant defense bonuses to an ally or take their place when they are attacked, etc. Certain combinations might also produce that kind of effect, like being able to divert attacks to yourself and then abort them with forced movement or something. No doubt MwaO can give you a whole recipe for something like that.
You might look at Insightful Warning - it gives +2 to you and all allies in a burst attack as an encounter basis - it is an Arcana Skill power. Powerful Warning, Bolstering Insight are a couple of interesting Warlord powers.
In general, though, a good defense tends to be more expensive than a good offense and a perfect defense usually ends up rewarding the DM by encouraging focus fire attacks against anyone but you. Or prolonging combat. Etc...
I tend to really hate boons, just as a head up.
LoL reflavored items, thanks
Item powers can have keywords, and 'stance' is just a keyword, so its perfectly feasible. It seems a little weird that it deflects things like fireballs and such though. I mean, mechanically that's cool, but just pretty odd from a flavor perspective. If its meant to be something like 'Arrow Cutting' then obviously it would only work on 'missiles'. In that case you might restrict it to single-target attacks, though that might make the whole thing not really worth bothering with. I don't know. .
Lets say we did go with only versus single target attacks instead of area effects how could we keep it worthwhile?
Well perhaps the stance can allow the user to split the defense gained between them and an adjacent ally....
Perhaps once a Day you can actually reflect the attack
Sure, you can 'cut arrows' aimed at anyone in your aura. Deflecting an attack would be cool.
Sure, you can 'cut arrows' aimed at anyone in your aura.