So, I finally figured out how experiment interactions work. It's a bit unintuitive, since all generated interactions mix them all together, but it IS possible to separate them.
Any interactions that involve animating or resurrecting corpses can be used in worldgen.
[EXPERIMENT_ONLY] means that an interaction cannot be used during gameplay, but if it involves either animating or resurrecting corpses, it CAN be used in worldgen. It does not require NIGHT_CREATURE_EXPERIMENTER or I_SOURCE:EXPERIMENT. Animated corpses can be used to construct towers, as usual.
[NIGHT_CREATURE_EXPERIMENTER] means the creature will occasionally "perform horrible experiments" under certain circumstances in worldgen.
While performing horrible experiments, a creature can use any interaction that has the [I_SOURCE:EXPERIMENT] tag, whether it is one of that creature's abilities or not. In fact, as far as I can tell, adding it as a creature's abilities doesn't actually do anything, despite the fact that generated necromancers have it.
So it is actually possible to make a secret that allows a creature to use specific animation or resurrection abilities during worldgen, without creating random generated experiment creatures. Just give it EXPERIMENT_ONLY interaction that either animates or resurrects corpses, and to prevent generic necromancers from creating the custom creature, leave out the I_SOURCE:EXPERIMENT from the interaction. However, this cannot be used to target live creatures with specific effects, since this will only occur during experiments.