I have also read on here from somebody that they contacted Universal and were able to get in touch with the head chef of Mythos at Islands of Adventure. The husband wanted to do something very special for his wife for their anniversary and the head chef created an entire 3 (or 4, can't remember) course meal for them custom made with food they both enjoy. They even has custom made menus and all the food was brought to the table by the head chef.
It never hurts to ask, but like this man, make sure you know what you are looking for. He knew he wanted a special meal at Mythos for the anniversary because Mythos was her favorite place to eat. Don't call Universal and ask what they can do for you, but call and ask if they can help you do what you want to do. Maybe your nephew really likes Shrek or the simpsons. Ask if Universal would help with a surprise by having them show up at a dinner that you had planned for some one on one time. Or you could call the hotel and ask if you could mail some items to them to place in the room before you checked in for a surprise. Just like Disney, Universal wants to make each guests stay a memory that will last a lifetime, so if they can help in a small way that doesn't cause them a lot of headache or to go out of the way to much, they should gladly help. If you call and get a "no", you can call again another day or ask to speak to another person.
Please note, the hotels are NOT owned by Universal. So, if you want a surprise in an onsite hotel, call the hotel's phone number which you can most easily find on the Loews hotel website. If you want a surprise like a custom meal at Mythos for example, call Universal direct and they will be able to direct your call to the person that can help you the best. Again, make sure you know what you want to do before calling so Universal can direct your call to the correct person. If you want something special at a Citywalk restaurant, call that place and not Universal.