I looked in https://secure.phabricator.com/conduit/ to the "differential.*" methods, but none seem to be usable to get the people associated with a diff and whether they approved or rejected.

For example for D17598, going to https://secure.phabricator.com/conduit/method/differential.revision.search/ and using queryKey {"ids": ["D17598"]} outputs details but no "chad" or any info about users/people.