I've seen an 'All Ghoul' undead warband, which proxied easily enough. A Strigoi Vampire with massive claws which counted as a pair of sword or axes (i forget which). The 'dregs' were armed with primitive versions of weapons (like a 'club' that was obviously a man's leg). A suitably depraved-looking necromancer fitted right in. Ghouls for henchmen, & an extra-mad ghoul on all-fours counted as a Dire Wolf. So I think that'd probably be a good fit for cannibals too.
I think you've about got it right with the No Pain rule; I did the same thing for a 'Masochist' character in a Slaaneshi Cult warband. 'Immune to Psychology' is covered by them being mad cannibals -- so is 'Cause Fear', because who wouldn't fear the flesh-eating maniac? I think 'Immune to Poison' can be explained through something like "Continual consumption of the foulest meat has left the warrior with a constitution that can survive anything. The warrior is not affected by any poison."
In this situation, I suppose the 'heroes' would mostly be those cannibals who still have the mental strength to use even basic tools, which could explain why the Necromancer & Dregs don't get those special rules. If you used Zombies, I suppose those are the maddest, shuffling degenerates, oblivious to anything except the smell of meat. Dire Wolves could be replaced with the Wardogs from the Witch Hunters list as a less overtly undead substitute. OR, you make them terrible tormented creatures the cannibals have... maybe stolen from a noble's menagerie, & now use as attack-beasts. Spoilers for Brotherhood of the Wolf, but like in that film.
Citizen Sade's suggestion of the Survivors of Strigos seems a good one too; it's largely an adaptation of the Rulebook Undead list, adjusted for a savager breed of Vampire. It also has the advantage of moving away from Necromantic Magic, which would be tough to explain in a non-undead cannibal warband.
The only alternative that I can think of just now is to use Orc rules, emphasising the brute-force aspect. Animosity rules reflect their tendency to turn on each other when hungry. But that seems a less appealing option, to my mind, than Undead (or Strigos), given you'd end up excluding a lot of the fun Orc & Goblin options, like Squigs.