Shadows and Outlines are handled by shaders/materials, and you can change materials mid-string with the <m=name> tag, after a material has been defined in the TextData inspector (the T icon in the top-right of every STM window). So you could use this to swap between multiple shaders!
To change a material after its been "drawn", you'll have to edit the materials/sharedMaterials array of STM's mesh renderer (supertextmesh.mr) component. The materials STM uses to draw text is a copy of the reference materials provided, so you'll need to change these materials directly through the mesh renderer.
