To throw out a wild idea here, who's to say you couldn't add some braces to reinforce the hood? If your engine is big, it might require careful placement in order to clear some components, but it doesn't seem like it should be impossible, if you can weld or know somebody who can.
The more concerning thing here is visibility, as already mentioned. Hoods are long and high on most dirt-worthy vehicles, which makes low objects such as pets, children, trash cans, shopping carts, compact cars, rocks, and tree stumps hard enough to see as it is. Efficiently strapping as much stuff to your car as possible should be less of a concern than the safety of others, others' property, and of course your own vehicle.
Then there is also the issue of cooling; unoccupied hoods conduct heat away from the engine compartment and, however minimally, aid in cooling. If you MUST put any thing on the hood, be aware that it will become extremely hot while the engine runs; and regardless of whether or not you add insulation to prevent this, objects on the hood will trap the heat and raise the normal driving temperature.
I say hard no to any thing on the hood, except when a hood scoop or cowl is necessitated... but those are usually for sports cars where visibility is less of a concern, and more space or more cooling is required.
