I remember reading a rather extensive thread on the board about a while back detailing just how many Gnolls or Orcs it would mathematically take to bring down a well built 20th level character and the number was obscene, in the thousands if not tens of thousands... and it got me to thinking - why wouldn't swarm rules be appropriate for humanoids when facing higher level characters.
They could be called 'humanoid' swarms. The rules would be much the same for them as regular swarms everything would simply be on a higher scale - the swarms would be significantly larger, covering 30 square feet perhaps, and damage would be ramped up considerably, but the concept would be the same - that there were so many overrunning you at once that single target attacks would be unlikely to deal meaningful damage and that there would be too many attacks to adequately defend yourself. Some would automatically get through regardless.
This also seems like an effective way to adjudicate scenarios when you're fighting hordes of humanoids, rather than rolling for a dozen attack rolls for every character every round, trying to figure out who gets what bonuses or penalties and who's flanking who, etc. its merely assumed that a set amount of damage gets through so long as the character is surrounded by lower level foes. Area bonuses or penalties (like Bardic Performance) would simply increase or decrease the damage of the swarm while in effect.
Stat blocks could be made for Orc swarms, Kobold swams, Goblin swarms, Gnoll swarms, even Ogre or Demon swarms. I would think that would make for some suitably epic threats. In our discussion, it was suggested that one rule that could be applied is that any martial character who took a full attack action while in the midst of a humanoid swarm would be able to deal damage to it equivalent to 1 standard attack which seems like a reasonable allowance.
3.5's DMG2 had a thing called the Mob template that pretty much did exactly this.