The inside surface of the buttons is flat, so if you wanted to you could put a label in there (I'm assuming you're thinking of using these as admin buttons). The inside diameter is 5/8", that would be as big as your label could be.
EDIT: Drilled a hole in the bottom of one and printed out a $ label and threw it together and lit it with an LED flashlight. They're kind of like a mini ring of light with a label in them. It doesn't seem that easy for the label to move once installed either, but you never know...