Here at the Naturalism and Ethics conference at Auckland and thinking about this again.
Christians seem to like stuff from Aristotle, so it puzzles me that I rarely see anything like the following discussed in contexts where it is asserted that there can't be ethics without God.
1. A thing that exists has the intrinsic nature it has whether or not God exists.
2. The conditions for an existing thing's flourishing are fully determined by its intrinsic nature.
3. How a thing ought to be treated is fully determined by the conditions of its flourishing.
4. Lemma: How a thing ought to be treated is fully determined by its intrinsic nature. 2,3
5. How a thing ought to be treated does not depend on whether God exists. 1,4
Wolterstorff discusses a Kantian "capacities approach" in his Justice book (HT Matt Flannigan) which is somewhat similar, but I think he gives it short shrift.