Additionally, the whole idea of antimuscarinic burden being clearly associated with delirium is on much shakier ground than you might think. The original paper that associated serum antimuscarinic activity with delirium has failed to replicate a few times now. Benzos and opioids are the only two things that I have seen consistently replicate as deleriogenic in the literature.
Plus, you know, the fact that quetiapine doesn't actually seem to make things any worse (though probably also not any better) flies in the face of the antimuscarinic hypothesis.
I wrote an essay on this specific topic, though I can't say I came out the other side feeling any more confident about what were supposed to do from a psychopharmacologic point of view:
Wot's Uh... The Deal With Antimuscarinics and Antihistaminergics in Delirium?