Man... I couldn't imagine doing that. Yes, you could, but the metal shavings will be stuck all over everything in your engine bay when you're done.

Why don't you buy a valve cover off of eBaY like I suggested? Those things go for dirt cheap. That way you can polish it, install it when you're done with a new valve cover gasket kit, and be done with it without the mess. If you haven't polished anything before, I definitely don't suggest you try it while the part is on the car.