Now Since 1.6.2 (July 2014)
This is possible for buttons, knobs and all other types of controls, too. See the manual entry here.
This has been implemented for labels but not for buttons. It's a separate control type, so you have to add your label that way.
Though it's tedious, you can put these on top of your buttons. All layering of controls in a layout persists over time. Remember, there's a "send to back" button in "actions."