Actually I do believe that McDonald's does have a grilled cheese sandwich, which is not on the regular menu but there anyway. You can ask for it, and the girl at the counter will blink a few times until another employee will remind her what key to press.

It will be grilled on the same grill that their burgers get grilled on, if you don't mind that.

Also some of their products, like the McRib, are mostly soybeans anyway. :)