Good luck parking at some of the resorts if you are not a guest...
If what Hammer says in the above post is true, I have one more, perhaps ideal, choice: 'Ohana's at the Polynesian...walking distance from (most of) the TTC lot
It's not exactly a buffet so much as all-you-can-eat family style fare. Your side dishes are presented on a lazy susan in the center of the table (and you can always ask for more), and the "entrees" are well-seasoned well-sized nuggets of various meats & fishes presented on spears by various servers. If they have food you want, they remove it from the skewer for you, and they keep coming back with more if you want it. They present fun things for kids to do, like races involving using brooms to sweep coconuts across the floor, and there is a view of the Castle from many of the seats (and I think they pipe in the music during the fireworks).
You might also want to consider the luau dinner show at the Poly. More expensive, but it IS a dinner show, plus it includes unlimited beer and wine. And again, being as it is at the Poly, it'll be one of the easiest restaurants to get to if they're prohibiting day guests from parking in a resort lot without a WDW pass.
If you try any of the other buffets or all you can eat fare at MK resorts (1900 Park Fare at GF, Chef Mickey's at Contemporary, Whispering Canyon at Wilderness Lodge, Trail's End at Fort Wilderness, you'll probably have to allot yourself more time to get there from the TTC, again, if Hammer is accurate.
Boma at AKL, being farther away from the action, might be more relaxed about day guests coming to visit. It's much less liely that someone will park there trying to avoid the theme park lots than they would at the MK or Epcot resorts. But make reservations just to be on the safe side.
Finally, I'm not a big seafood lover, so the Cape May Clam Bake Buffet is wasted on me. But their breakfast buffet kicks a significant amount of keister, If breakfast is an option for your get-together, I recommend it, too.
