my dad will be celebrating his birthday on the ship next week when we sail, and we just simply ordered the regular birthday cake...
I think your best bet would be to wait until you are on board and have a talk with your head waiter to see what you have in mind and what can be arranged... To be honest, the desserts are so good there, I can't imagine wanting another specialty dessert made on top of that. I think that is why they just stick to the cakes...although I am sure any one of their delectable desserts can certainly be made more festive with a candle (or two!) and the Disney waiters singing happy birthday!!!