Jessica:
This is a good question since only guests who stay on property are allowed to participate in Disney's Extra Magic Hours program allowing guests to either come early before the park opens or stay late after the park closes. And because you are staying off-property, I can understand your concern.
HOWEVER, when I looked at the park calendar for August 15 for Magic Kingdom (click
here), it isn't showing Extra Magic Hours for that day. The park is simply open late until 11 p.m. for all guests! So you're in luck!
I'd stick to your plan of going early, taking a break, and staying up until it closes! Best of all, Magic Kingdom DOES have Late Magic Hours the next day on August 16--and, as an official resort guest by that time, you'll be able to enjoy them!
Wishing y'all a magical vacation!
Whitney