This cake is awesome! I have been trying to visualize how you were going to do it after first reading your post when you were contemplating whether or not to do it... it's great! If you have time, you can always cover the white in white stars, like the character pan cakes are iced. This would cover any bare spots and also add a different texture. The color mist spray is also a great idea like was mentioned by another poster. I also like the "Happy Birthday" idea that was mentioned.
However, as many others have said, the majority of the people who see your cake are going to be saying "WOW!" and are going to be so impressed by the enormity of the whole thing (size, design, talent, time involved) they aren't going to think anything of not-perfectly-smooth icing! The cakes I've done that I really thought were awful (mostly due to un-smooth icing) were the most appreciated cakes.
You are doing an incredibly wonderful job! Keep smiling, put on some soothing or dancing music, whichever works best for you, and go it!