Once upon a time...
There was a company that supplied what amounted to rear lens caps with 1/4" x 20 screws which were designed to mount into the tripod screw socket beneath the camera...
The rationale of this was that you could carry your camera with the shooting lens mounted on the camera with the extra lens mounted beneath the camera.
The idea never got off the ground. However it would be easy to fabricate one from an extra lens rear cap and a 1/4" x 20 screw..
My fear would be, that besides making the camera awkward and heavy, there would be a possibility of the lens becoming loose and falling to the ground. Imagine your chagrin if your new 17-55mm f/2.8 IS lens landed front element first on the sidewalk.
How do I shoot? I use two cameras on an OPTECH Dual Harness and this works perfectly...
http://www.optechusa.com/product/detail/?PRODUCT_ID=87
