Well, there's a barfight, and there's a barroom brawl.
I barfight is a couple of guys (or at least pretty equally numbered sides) going toe to toe and is pretty controlled, straight forward combat.
A brawl is when all hell breaks loose. In my opinion rules go out the window (the game kind and those in the pretend world), as weel as bodies, and you can pretty much even just narrate the goings on.
Who says that a thrown bottle or bench can't deliver lethal force? It's like navigating and asteroid field: Something is bound to hit you eventually and there isn't a damn thing you can do about it, it jsut a matter of how big.
I'd actually remind the players (nah...I'd actually teach them the hard way) that consecutive rounds in combat in a brawl really translate to the fact that they aren't paying attention to everything else. That's when people get hit in the head by errant flying plates, get stumbled into by a wayward grappling match, or even taking a dagger to the gut in a case of mistaken identity.
To simulate this, I'd rule as follows:
1) To determine if a PC is struck by some of the random ruckus around him, I'd roll an attack die at some reasonable amount of bonus for the party level for each round that the brawl persists + the number of combatants. In other words, if it is a brawl in the 6th round of duration, and there are 17 people brawling, you'd roll 6d20 + 17d6. Start with the highest die roll being distributed as a successful attack against the lowest DV in the room (bystanders included). Keep going until you run ouf of these attacj dice, then move on to the next round.
Players can toss thier attacks in themselves, but unless the keep to fighting one opponent each, their attacks jsut jumble into the pool.
2) Each round after the first that a PC remains preoccupied in a narrom brawl fighting on his own, damage dealt to him is considered to be 1d6 of cumulative Sneak Attack damage, regardless of the source. So, if the brawl starts, and one of you PCs keeps trying to knock out the same guy in front of him for three rounds, and damage that he takes outside of that little combat deals +1d6 Sneak Attack damage. Yes, it would stack with other sneak attack damage.
Probably needs some refining, because that's jsut off the top of my head, but you get the idea. There's a reason most people bail out of barroom brawls when the set of!!!