just create another sprite as per desired shadow effect and spawn it behind the object when needed... is a sloppy job, but that's the only quick way of doing it in the canvas element without "technical" difficulties
i think text objects in C3 support shadow effects ...... bbox codes if im not mistaken .... search the forum for text object bbcode it should take html css styling therefore making it shadow possible.
only in C3 this might work... C2 doesn't do that... only if you are using a text input and make it look like a text object using CSS.