You probably can't.
If there are a small number of different dock labels you could define one for each in the stylesheet.css and then use the style selectors to attach them to the correct one.
If there are too many to manage this way (or the text is dynamic) then you need a different approach.
One possible approach might be to put a label in the dock that shows the text that you want. Then use styles and css to offset it to a position outside of the dock. You will need to set the clip behavior so that it is shown even though it is outside of its container.