I've done this several times, not for myself (I have an annual pass) but to bring guests into the park for lunch.
The big advantage of eating at Mythos on a day you're not touring the park is that you do not have to take valuable time away from riding the coasters to eat a big meal. And Mythos is something of an attraction in itself. If you are having a real meal -- i.e. appetizer, entree, and dessert with wines and all that -- then you can easily spend two hours there.
The way it works is you make a phone reservation at Mythos (407-224-4534), then go to Guest Services and tell them you're going to Mythos and you want a Diner's Pass. They'll take an imprint of your credit card and if you aren't back in the alloted time to collect your credit card slip, they'll charge your card for full park admission for the number of people in your party. (BTW, if you find yourself running late, just tell the server and they'll cut you some slack at Guest Services.)
Every time I've done it it's worked flawlessly.