If you size the chain for Big-Big, you will be fine, and the worst case scenario is that your chain goes slack in the couple smallest cogs if you accidentally cross chain. And that will make enough racket that you will know it is happening.
But really, the only reason to do this is if you already have the RD. If you need to buy one, there is no reason not to get an RD with the proper capacity. This is especially true if you want the bike truly idiot-proof. Because idiots WILL cross chain, and a bunch of noise from the RD is not going to stop them.