Okay, after reading Untouch's post about newly added npc entity, I decided to do fome testing. Here is what I found:
1. Unlike mvm robots and tf2 bots, these... things... are counted by engine as NPCs, not as players. What means that there can be more than 32 of them in one map.
2. When spawned trough a console, they appear with a random "voodoo soul" model (hovever, I think there is a keyvalue for the model, like in scientist entity in half-life). It doesn't matter what class skin they have, they all have 100 health and run about as fast as a medic.
3. The spy model is broken. Just a funny fact.
4 The ragdolls they drop after death are broken as well. Exept they are LITERALLY borken. I can't describe this, you need to see this on your own. It looks like valve wanted to make some sort of.. gore effect? Dunno.
5. After spawning they just wander around until they saw you, at with point they run at you with meele swing animation. Looks just what I would expect zombies to do. And of course they punch (?) you when you are close enough, what tooks away like... 15 health?
4. They dont have hitboxes. So you cant headshot or backstab them. They also cant be set on fire, or damaged by heavy's new chineese minigun's ring of fire. Flamethrowers and flareguns works as intended (exept that, of course, they can't set them on fire).
5. They can target cloaked or disguised spy. Boo-hoo.
6. Sentries target them normaly. Don't know why, but I rather expected them to do otherwise.
7. They are immune to critical hits and mini-crits as well as damage drop-off and ramp-up.
8. Granedes don't blow up on touch. Sorry, Loh 'n Load.
9. They not affected by ANY kind of knockback. However, they can jump just well.
10. They don't destroy bulidings. Just... don't. They doesn't even seem to notce them. If you want to create a mvm map with them, I would reccomed you to put some sentry busters in pop file. Otherwise it will turn into engineer slaughterhouse.
11. They work in any gamemode. Thats right, unlike upgrade stations and robots, this new feature will work in any kind of map.
If this info is all true it sounds as if Valve were planning a more adaptable version of the robots for the zombie MvM map, and they most likely didn't get it finished in time. Which explains why the current mode feels rushed.
Shame, would have been nice to have Wave 666 with so many more enemies on screen at once, as well as having enemies which actually sound, act and look like zombies, rather than robots wearing Halloween costumes.
I assume with a entity_maker or what ever that entity was called. Just let it spawn a tf_zombie and it should work. The map needs a nav file though (this wasn't mentioned in the OP, was it? :o)
Sometimes I play with a few friends on a private server and just let them spawn so we can kill some time along with the zombies.
if you want to enable them on your map you need this commands:
(might be spelled wrong or just use other words, i dont have them compleetly in mind right now)
tf_force_halloween 2 (to enable halloween mode, somehow they only work when this is set)
tf_halloween_zombie_mod_enable 1 (they now spawn)
tf_halloween_zombie_xxx (there are a few commands to set the time between the waves, the amount of zombies in a wave, speed and HP and damage of a zombie.)