I like how aperture has a dedicated button. I would expect ISO to have one too. It requires you to press the middle button among five buttons (there is a little bump on it so you will be able to know without looking) without spinning the dial, so it's slightly troublesome. I will prefer something like Nikon's mechanism, with two dials.
You can set up the 6D so that you can press the 'Set' button and use the top control dial to change ISO in shooting mode. This has a second advantage of automatically activating the viewfinder head's-down display, which pressing the ISO button does not; if you are trying to do it with the camera to your eye and want to see the changes in the viewfinder, you have to half-press the shutter release.
The quickest way to set this up is probably to open the Q menu, then select the 'Custom Controls' icon (the little camera with the lines to the right). Press 'Set' to open the menu, then scroll over to the Set button icon at the top right. Press Set to open the menu for the Set button, then scroll to the ISO selection. Press Set one last time, then close the menus (press the shutter release halfway is fastest way). Now you can press the Set button when you are shooting, and use the top dial to adjust ISO! The only problem I have with this is my hands are small, so it is a stretch for me.
Something else I use is to set the ISO steps to 1-stop rather than 1/3-stop, which makes it quicker to make changes in the ISO.
Now if they would just allow you to set white balance to some button!
Hope this helps!
Very interesting technique, I might try it. I mostly just set the ISO to auto, because there isn't a noise penalty.