Wait, are the creation of ghouls and resurrected undead recorded in worldgen? Now that I think about it, in worldgen I've only seen mention of "Necromancer changed victim into a XXXX" or "Necromancer resurrected victim as a XXXX" where XXXX is a randomly generated monster species, not a condition applied to a regular species (like ghouls would be).
I haven't done a whole lot of actual in-game exploring, only searching through worldgen. (I did once go to a tower of a modded experimenter and found no modded ghouls, but maybe that tower just didn't happen to have any). People have reported seeing generated ghouls in-game though, right?
EDIT: No, wait, I have seen necromancers create generated ghouls in worldgen. But I don't think they don't show up in world sites and pops exports.
EDIT2:
Toady looked at the issue and maybe it does work, but with some caveats? Going to test it.
EDIT3: Even with the fixes I still haven't seen it work, though he does say it might be rare...if anyone actually sees a modded ghoul or resurrection experiment, can you mention it?
EDIT4: Okay, finally got it to work. It's pretty rare though - you'll need to have a lot of people learn the secret before you have a good chance of finding one that actually starts making ghouls, but once they start they tend to make quite a large number of them.
EDIT5: It looks like there is an event where a unit will "perform horrible experiments" in a location. Once they do that, they will produce many experimental interactions. Intriguingly, this does not appear to require a tower! Now the real question: what determines the occurrence of this event? If it is the unit's personality, it may be possible to increase the likelihood of this happening. I will go on an adventure to meet these units and see if I can discover anything notable about their personalities.