Do Yorick's ghouls and the damage they deal count as ability damage, and specifically, would they trigger the rune Ravenous Hunter, and heal me based on the damage they deal? I can not find conclusive evidence.
Ravenous Hunter is a Drain Effect, so its apply to all damage sources.
Liandry's Torment has 2 passives: Madness stacks w/ any damage you deal; Torment is applied by any Ability Damage, so if your ability deal damage to a unity torment will proc.
Yorick's ghouls do apply spell damage, so yes it works.