This happened in FLG'72 IIRC, though it was not in the same manner that the thread suggests.
I think it would be easier to prevent Benn and Foot rising to prominence in the first place, and have there roles played by other left wingers who are more inclined to defection.Even though one of the ideas we've been going for is a 1963 PoD with having Brown succeed Gaitskell over Wilson, itself a key suggestion of the long-term decline in the influence of the Labour left, would the groundwork to allow such leading lights as Foot, Benn, etc. need to put down earlier still? Or is that putting forward too much work altogether?