I've heard this can be done on some ships, but not during peak times. One maitre d' on a smaller NCL ship told me that those wishing to reserve a table or waiter will have to dine before a certain time, say 5:45, or after a certain time, such as 8:30. This will vary from ship to ship, so check with the maitre d' when you board. I'm not promising it can be done, as it is ultimately up to the head maitre d'.
On what ship is your cruise?