Sigh, I found out. Apparently the answer is no, Mist is not Baton Passable.
In Generation II, Mist is a user-exclusive effect. It can be Baton Passed.
From Generation III onwards, Mist applies to the user's entire side of the field, meaning it cannot be Baton Passed (and would serve no purpose even if it could).
Mist (Bulbapedia)