I'm not sure off the top of my head, but I don't think it's safe to assume the the text in labels works the same as textures on nodes.
you could render your text into a viewport/render target texture and then load that onto a sprite or control node and do the shader magic, but it would come at a performance cost. Maybe someone who has used labels more extensively can offer better advice though since I most certainly would expect either the label or rich text label especially to offer some non shader based options for dressing up the text in your game/application.