You could hang the lens strap (not the camera strap) around your neck and let the weight rest on the strap. Then make the adjustment to focus settings. Alternatively, you can put the lens on the ground so that the lens hood contacts the ground. Finally there is a custom setting which changes the way the button+dial controls work, allowing one to first press the button with one hand and without holding it down, rotate the dial, finally press the button again to end the setting.
If you have ever tried walking-and-stalking birds in African thornveld, you would appreciate that none of these methods are practical. Apart from the strap, they simply take too long. I wish I could re-program one of the right-hand-side buttons to take over this role.
As for the strap, it is in the way 90% of the time. Certainly this combination is too heavy to carry slung for extended periods. Thus you would have to sling and un-sling continuously, landing right back at the point where it just takes too long.