Just ask when you arrive. I have had a birthday cake done for my husband each year because we always go during his birthday. They are always very accomodating. We had a cake delivered in CN at dinner - the servers sang, it was great! At CTI they delivered a cake during the repeaters dinner! I attached a couple of pics.
send a note to conciere, request what you want etc and they will forward you the info for payment. I had one ordered for my husband for his birthday this past December and it was delicous! Cost was $30 but well worth his surprise!
We are always at CTI for my hubby's birthday and while I haven't ordered a cake, the staff has always done something special for his birthday. I am sure if you let them know, they will do something special for your wife.