When DH and I were picking out our wedding cake, we wanted to go all sponge, but the cake lady chastised us, saying that the tradition is to provide a cake that your guests will enjoy, as it's your way of "giving back" to your guests. (I think that giving them dinner and a party was enough, but whatever.) So we went with 3 layers. The bottom was white sponge with vanilla buttercream, the middle was chocolate sponge with fudge, and the top was the fruitcake. That way, everyone got what they wanted. Everyone thought it was cool that they could have chocolate wedding cake. It had that hard white icing, and I thought that it would be gross, but I was pleasantly surprised when I tried it.
Anyway, have you checked with many local bakeries and/or wedding-cake makers? We found that most places were surprised when we said that we wanted a sponge cake for our wedding, but they were all more than happy to oblige.