I’m using the php date format inside the calendar feed setting in the admin dashboard.
It isn’t a “hack” as such. It’s supposed to be the way you output literal characters inside a formatting string in a PHP date string. It’s in the documentation.
It looks, though, like I can do this to do what I want:
[start-custom format="M"]<span>[start-custom format="j"]</span>
…to get what I need. It’s probably also easier for someone to look at and say “Oh, that’s what it does.”