I get the sense that they will not actually be staying on property at the time of their anniversary.... If that is true, you might contact the resort at which they stay most frequently and ask if it is possible.... not sure if they will or not, and it may depend upon the specific resort. Can't hurt to ask.
DH and I have received these greeting cards from both Mickey (DH's Birthday) and from Mickey and Minnie together (our 27th anniversary). We were staying at the BCV at the time and they appeared in the room with balloons on the respective days (which I had noted to the CM at check-in). OTOH, we stayed at WL for our 25th, and noted it when making the ressie as well as at check in, and no extra magic was provided other than that which came during the course of our trip (which was great, anyway).
I guess the bottom line is, go ahead and ask, but there are no guarantees.
