The Wishes Dessert Party is held at the Grand Floridan I don't know the minnimum but I think it was kinda high and to pipe in the music is $500 or $800 (i've seen both numbers recently so I don't know which is correct) so that might be costly for a small party, but you can rent the Grand 1 or a pontoon boat and have snacks on those.
Information on the boats:
http://www.wdwinfo.com/recreation/fireworks.htm
MAGIC KINGDOM WISHES PREMIUM CRUISE
25' Tracker Pontoon
-$319.50, tax included for up to 10 guests. Price includes pontoon boat, driver, water, soda, bagged snacks and AUDIO FEED
-Guests may order Private Dining to be delivered to the cruise
VIP Cruise on Grand I Yacht
– a 45-foot Sea Ray yacht staffed by a captain and deckhand, 17/18 guests maximum, available at Disney’s Grand Floridian Resort & Spa, $480/60 minutes, $720 for 90 minutes, $960/2 hours . Must call (407)-824-2682 to book.