I was always under the impression that the hypothetical cheeseburger could potentially be eaten, provided the ring-user used enough willpower to will-into-existence the various ingredients, down to the atomic level. So you would have to be extremely, impossibly intelligent, but it could be done. The only problem is, once you discontinue the ring-construct of the cheeseburger, any nourishment you get from it would disappear from your body.
My understanding of it is that the ring is literally only limited to the user's imagination and willpower. Giving the rings specific boundaries (beyond the whole yellow thing) is silly to me.
Was this ever explored in the book? It's an interesting concept and might make for a pretty epic story but it's not something I'd want to see GLs doing all of the time. I'd never even heard of this being possible.