I've screwed around with treating mobs of mooks as a circumstance. You see panels in the comics where the heroes are fighting their way through a legion of mole-men or robots, smashing, and hewing, but what they're really trying to do is reach The Slayer of Time before he can activate his Anti-Chronometer and destroy history. 

The swarms of baddies just make this harder... so, an evil "Swarm of Android Rosie O'Donnell" might be statted out like so... "6/2A" This means, you either make your roll against a difficulty of 6 or you take Area dice of damage to ignore the diff for one round (regardless of how tough you are).

Heck... brain bubbling...

MENACES
Is is DEADLY? (inflicts shock and killing to buy off the diff)
Is it ESCALATING (each roll which fails because of the Menaces Diff adds either 1 die to the damage or 1 point to the Difficulty)
Is it PERSISTENT? (can only be evaded for 1 round, then comes back)
Is it BIG (affects everyone rather than a specific character)

Your basic Menace has a diff and inflicts 1 Area die of shock damage.

A menace complicates another roll with its Diff. A character can deal directly with a Menace by overcoming its Difficulty. If the Menace is Persistent, then this only reduces the Difficulty by 1, meaning a series of successes is needed to defeat it completely.

A Menace could be a rock slide, a giant stone ball, a bunch of pissed off hyper-gorillas, or whatever. Anything that distracts and complicates the characters main goals.

Menaces are built with WILLPOWER. 1 point per level of Difficulty, 1 point per area die of damage per Quality. This total Willpower is then shared when the Menace is defeated.

Example Menace:
Enraged Hyper-Gorilla Army 5/3d
Is is DEADLY? --- Yes
Is it ESCALATING? --- Yes
Is it PERSISTENT? --- Yes
Is it BIG? -- Yes

They watched Gorillas in the Mist and King Kong, and now they're pissed. They're worth 40 Willpower if you can take them out, but that's going to be pretty damned hard. It's a Diff 5 to do anything while also fending off the gorillas, and trying to bash them into submission is doubly tricky because if you blow it, they get madder and nastier, and if you make it, you only moderate their fury rather than abate it. If you take damage from them, it's shock and killing and starts at 3d. A couple of tens could take you out. Finally, the army is big enough that you and all your buddies are going to be stuck right in the thick of it together.


Alternately...

Rather than do the passive Difficulty and Damage thing (which I like, because it reduces GM rolling), use dice for these things, and roll them and use the sets to gobble player sets (the complicating business of dealing with maddened gorillas while trying to deprogram a doomsday device) or treat them as attacks (rocks fall, someone might die).