By being prepared, that implies that you are carrying at least a sleeping pad and sleeping bag, and possibly a bivy/tent shelter.
If a ranger checks your permit, and sees you are carrying that equipment, I think he would believe you are planning on spending a night, and therefore would expect you to have an overnight permit. I'm just not sure how it would go trying to convince him that your gear was only there in case of emergency.
It would be better if you just obtained the overnight permit, and then if you completed the hike in a single day, there would be no problems. In fact, if you go prepared for an overnight, you will likely be carrying more than 30 lbs, and that pretty much guarantees that you will not make it in a day.
So... when you go to pick up your day hike permit, it is quite possible that there will be overnight permits available due to people dropping some of their slots. Just ask for the overnight permit.
Not sure if they will give you a parking pass for the overnight, since they are only giving those to people with proper reservations.
