Yes, that sometimes is what happens at the hotels....the wedding dinner/reception is in one of the hotel's a la carte restaurants. I've had dinner at one of the restaurants one time while the wedding party was having their dinner. But I will say that the staff sectioned it off for them so that no one mingled or mixed in with them and did a great job keeping it as private as one can for them.
You can contact this individual named Pototo who lives in Cuba. He is well-known to help visitors and knows the areas. He may very well be able to help you with regards to finding a place to have your party.
Here's the link and contact info:
http://cuba-junky.com/havana/pototo.htm
http://cuba-junky.com/email-pototo.htm
Another thing you may be able to do is rent a villa in Cuba for a day.
http://www.cuba-yes.com/villas-cuba.php
Beardo also suggested Josone Park, which is a great idea. According to this information, the park covers nine hectares of grounds, most of which have been left in their natural state. There are four restaurants-Antigüedades, Retiro, Dante and La Campana-specializing in international, Italian and Cuban cuisine, with cocktails, meat, fish and shellfish; an establishment selling sugarcane juice (which is extracted from the sugarcane while you watch); and a bar, La Gruta, on the shores of a lake.
You can explore the park on foot, by bicycle, in a horse-drawn carriage or by boat. It has a swimming pool and a party room for children (with dance, music, painting, drama and singing classes; performances by clowns; and productions put on by children). This is an ideal place for banquets, business dinners and parties. In addition, you can rent lockers here for when you go swimming at the beach. For all these reasons, Josone Park is called "a green paradise inside the blue one."
http://www.supercubatravel.com/destinati…
So there are facilities that may be open for a wedding banquet.
Congratulations and all the best in your wedding plans!