I think Burning Wheel works great for this, actually. The other subsystem you’re thinking of is Bloody Vs. For Fight! you can make liberal use of the helping/outnumbered rules to run 6 vs 4 pretty easily. You can also use Bloody Vs to abstract such a fight out a little more. Bloody Vs uses split dice pools where participants split between attack and defense, as well as a bunch of factors taken into account for advantage dice.
I’d recommend starting with Bloody Vs for group combats and easing into Fight! with some Trial-by-Combat style duels if you can.
Someone also is/was working on an ASoIaF hack for Burning Wheel; I’ll edit a link in here when I can find it for you.
Seems I was somewhat mistaken, but these folks have run games in the setting and have done a little bit of hacking to do so; they may,be able to offer some insight, at least: