Shoot me down in flames if this is completely off track, (I have scanned the Baez material briefly), but the mention of classical vs quantum spin brings to mind the work of Levy LeBlond and the discussion of whether spin is an SR as opposed to Galilean Relativistic effect and more recently even a classical (non-quantum) effect ?:
the work of Levy LeBlond and the discussion of whether spin is an SR as opposed to Galilean Relativistic effect
and more recently even a classical (non-quantum) effect.
There is also the earlier SE thread Dependence of spin on classical vs non-classical physics?