So, in answer to my own question, it appears the reference is *The Theory Of Committees And Elections* by Duncan Black. Also perhaps relevant is *Theory of Voting* by Robin Farquharson, although I haven't got my hands on it yet.

Moreover the theorem is less powerful than what is stated above, and in a sense totally obvious. The theorem is, in modern terms: if there are a finite number of amendments to be voted upon, with no repeat votes on an amendment, and the last amendment winning, any amendment in the Smith set can win, by varying the order in which the amendments are voted for.