This won't work really well. As you can see, Bitcoin is currently suffering super slow confirmation of transactions, because of its congested network. The more Bitcoin become popular and widely used, there will be a lot of transactions that will pile up to Bitcoin's blockchain, which will cause a massive chaos within the Bitcoin network. We know that McDonalds is a fast food restaurant with a fast pace style of environment and that is one of the reasons they became pretty popular, "Serving your food, seconds after you ordered". I couldn't imagine someone, ordering a food, handle his / her BTC payment, then will wait for a whole 30 minutes or worst 1 hour just to get the food because of an unconfirmed transaction. This might ruin what McDonalds has established.