That is a similar method used to mount movie cameras on vehicles. The ones you've found are obviously smaller, but then again so is your (D)SLR, even with a battery grip attached.
Think your 5D is precious, try mounting 100+ pounds and several hundred thousand dollars worth of 35mm Pannavision camera on the side of your Holden.
The big brother version is called the Super Grip mount. A quick google search returns lots of links, such as:
http://www.alangordon.com/s_tripods_supergrip.html
Another method is to use a device called a Hostess Tray. Think of the tray you'd set on your car window at the drive-in burger joint, add a suction cup or two and come good, strong ratchet straps and you get the idea.
I'd still test it with something other than **MY** Nikon. anyone want to loan me their Canon? <<DUCK>>