After reading Mark's comments it seems like a good approach, knowing that a moderator has explicitly chosen to "always accept" a post from a user, would be to first create that user record (including setting the moderation_action property to defer—as Mark indicates) , before approving their email. That way when the message itself is approved the user record is already included. And then we don't have any need to explicitly update the list's accept_these_nonmembers property because we've already set the moderation_action on the user.
Does that sound like an acceptable approach?
And a follow up question: at the moment we don't explicitly create users, we simply subscribe them to a list, and allow the implicit processes to do their magic. However if I wanted to manually create a nonmember, would I "create a user" or would I "subscribe a user to a list" with a role of nonmember?
Which brings up an additional question. Is the moderation_action list-specific or does it apply to all lists from the same domain? To be clear, if abc@123.com's moderation_action is set to "defer" for **open-list@foo.bar** would that setting also apply to **another-open-list@foo.bar**?