i thought about it also but agree with first comment that it looks a bit silly, and no matter how carefull you are little stone chips will get under there,,, you cant stop that and you know whats going to happen then dont you - yep continuos scratching until it comes off or falls out.
touch up pen is all we can do and maybe if the car is worth it a bonnet respray every 3 years or so.
plus i think it will discolour the bonnet after a while with the continuos sun on only one part,,, that would be a nightmare as it would also effect where the bra wraps around the car, not to mention the different salt / dirt effects of different areas covered and not covered.
hope this helps