Hi Chris,
Great idea!
My family love to watch Wishes from the beach at Disney's Polynesian Resort. That combined with dinner at Ohana's is pure perfection!
The park calendars will not be available for September 2012 until the beginning of March 2012.
Once March 2012 has arrived, you will need to check the park calendar for the Magic Kingdom on the Walt Disney World website which you can find
HERE. The park hours are always displayed for the current month but if you click on the drop down box next to 'View Month' you will then be able to select September 2012.
Once the hours for September 2012 are being shown, if you click onto the individual dates of your visit, you will be able to see if Wishes is scheduled to be shown on each day and if so, at what time.
My family like to visit Walt Disney World during September and based on previous trips we have taken at this time of year, I would say that there is a good chance that Wishes will take place at 9pm on select nights during your visit.
If this is the case, I would recommend making an advance dining reservation at 'Ohana for 7.15pm on the evening of your choice. This will give you plenty of time to enjoy your meal as well as the entertainment on offer at the restaurant before making your way to the beach to watch the fireworks. If there are any hammocks available grab one, if not, find somewhere to sit on the beach itself, relax and enjoy the show.
Wishing you a magical holiday!