@RMiddleton It's actually a fascinating study. You're right that it's nominative/accusative almost always, but in the case of pronouns that don't have an accusative form, the genitive will be used instead (ie for it/its).
Or, if someone has two sets of preferred pronouns, they'll put them both in there, with the secondary either in the nominative or accusative. He/her and he/she are both used, for instance.