Is there any way to get the mighty swing from the war hulk prestige class to work so the Knockback feat affects all targets?
Mighty swing works like great swing, and great swing says this:
If the war hulk uses a special attack (such as disarm, trip, or sunder), this special attack affects only the first target; the other creatures are attacked normally.
And since bull rush is a special attack it won't work, I think, so is there any way to make it work?
So bull rushes aren’t, generally speaking, special attacks the way disarm, trip, or sunder are. That is, you don’t make a bull rush in place of an attack; a bull rush ordinarily requires a standard action.
With the Knockback feat,
If you score a hit while you are using the Power Attack feat, you can make a free bull rush attempt against the foe you hit
Note the wording here: Knockback doesn’t change your attack itself. That attack is not “special,” Knockback just applies to a (conditional) bonus feature to all attacks. If you hit and are using Power Attack, then you get to make a bull rush as a free action against that target. In effect, the bull rush is “triggered” by the attack (which is a regular attack).
With this in mind, consider a great, mighty, or massive swing: you attack, and that attack applies to all creatures in three squares (or all squares around him, in the case of a massive swing). Assuming the attack hits each, and you were using Power Attack, you have scored a hit against each of those creatures. Per Knockback, each of those hits triggers a free bull rush against the hit target. So you get to make a separate bull rush against each one, but these are separate actions from your great/mighty/massive swing, and nothing about those features comes into play.
A DM could very easily argue that, despite being technically accurate, the above is against the spirit of great/mighty/massive swing, and nix the combo. If that happens, you are basically out of luck: there is no feat or other option that is going to change that limitation on great/mighty/massive swing.
The closest thing I can think of is to try replacing war hulk with war mind, and leverage sweeping strike, which only affects two squares but applies to every attack, and has no rule about special attacks at all.
