Your fears are absolutely justified. But, digging in…

The lens mount itself is bolted to the camera’s lightbox. In order to damage the mount, we would need to stress it to the point of damaging itself, the bolts holding it to the lightbox, or the lightbox itself.

The weight of the camera is important only insofar as one could infer the sturdiness of these components from the weight (plastic frame/aluminum/magnesium alloy). So, let’s disregard the weight of the camera and focus instead on the materials being used to support the mount.

Now, the lens mount can probably support a good deal of weight when pointed towards the ground. At this angle, any force will be evenly distributed along all of the connecting components. But I'd worry about the force being applied at an angle (as would be applied by pointing the lens parallel to the ground and not supporting it).

The farther the weight is from the mount, the more force will be applied to it, torqueing it downward. So, you have little to worry about with heavy, short lenses.

But, when using long and heavy lenses, well, that’s when I’d start to worry. I’d expect to damage lighter/less sturdy materials before more sturdy ones – ie I’d expect a Rebel’s mount to fail before a Pro model. I haven’t been able to dig up any actual fail weights – as not many are apt to want to destroy a camera for the test.

So, to play it safe, when using a large or heavy lens, always support the lens. Your left hand should support the weight of the rig - your right hand shouldn't need to support any weight at all. If the lens has the option of using a tripod collar, you should be using it with a mono/tripod as opposed to mounting to the camera body.

All that being said, lens mounts are very sturdy. You would probably need to be holding just the camera using a 400 f/2.8 to get anywhere near the force to really damage the mount. Doing something like this is either an exercise in science or stupidity. So if you do it, publish your results.