I'm sure each manufacturer has a specification for the amount of force along several axes that their lens mounts need to be able to accommodate, but they don't seem to publish that data.

They are probably not published because in the real world it is a specification that is not practically needed. It is fairly simple to understand why. Which to support (camera or lens?) is a decision based not on the load capacity of the lens mount, but rather on which allows the user to keep the camera/lens balanced and more stable while shooting. The transition to primarily supporting the lens should occur well before the limits of the camera's lens mount is reached. When lenses have less mass than the body they are attached to they should be supported by the body. When lenses weigh more than the body the body/lens combination should be supported by the lens. You do this not because of the load capacity of the lens mount, but rather to maintain balance and stability when shooting.

Lens mounts are not only built to support the weight of the pieces they hold together, but to handle the force those pieces can place on the mount when dropped from small distances. The static weight of a lens would need to be several multiples of the weight of a body before the mount couldn't handle it. Further, to the best of my knowledge the specification, for example, of the Canon EF/EF-S mount is exactly the same regardless of whether the camera in question is a Rebel SL1 (14.4 oz.) or a 1D X (54 oz.).