It also appears the both Kirk and ReallyRight have copied the Canon design without changes and consideration for the balance of the camera and lens when used on a gimbal or video head.
Not sure I agree. The Kirk foot does mirror the Canon foot's forward-sweeping design. But, the RRS does not - the dovetail actually extends back farther, directly under and even slightly behind the mounting screws:
Granted, it may not be quite far enough back for the 500 II w/ a TC and heavy body. The RRS 300 II foot dovetail extends substantially behind the mounting screws, for example. However, apparently RRS designed the foot for the 400 II (before the 500/600 II's became available), then just declared the same foot was compatible with the longer lenses, and subsequently backpedaled with their note on the website. Perhaps they'll release a new foot designed for the longer lenses - if so, hopefully they'll do right by those who bought the maybe-not-so-compatible current foot (me, included).