Thanks for the reply. It's good to know I wasn't missing something obvious.
Just to clarify, this isn't something I can currently do in Chipmunk? Can anyone think of a way to fake it? I was thinking of trying to apply forces perpendicular to the vector between the pivot and the object, and relative to the mass of the other object. But I don't think that's right, because it could result in an acceleration of the whole construct.