I don't know if they have this at the grocery stores with bakeries near you (like Giant Foods or Safeway), but when my brother ordered an anniversary cake for our parents, they were able to put a photo image on to the cake (for them, it was a picture from their wedding).
Anyway, my point, if it's not expensive, take an up close picture of a d20 on a "1" then have that picture put on the cake...
(maybe - well, at least it sounds good in my head).
edit: also don't know how much it would cost so it may not be worth it...