Wow.
Just.... wow.

For simple monochromatic recessed lettering, just skip to Step Two.

Step One: For Colored, Glow or Lighted letters. Make the show-through surface of your model printed in color, glow in the dark or clear (nylon, etc.). If clear, you may want an LED light source beneath that layer.

Print the Stencil Layer layer of solid color.

Step Two. The Stencil Layer. Make a surface with no thickness. Apply 3D Text using the Fill/Extruded option, set to the same height as you want the stencil thick.

Step Three. Select the surface, but not the Text, and extrude (Pull) it up to the same height as the text so it is flush.

Step Four. Explode the text.

Step Five. If your text has free standing islands, like the letters O, B, P or whatever, you will need to draw and erase lines to connect them to the outer surface ala stencil font.

Step Six. Select the inner surface of a text letter and Push it down either partially (for recessed monochromatic text or all the way for see-through stencil.

Save.

Someone smarter than me could probably write a plug-in to do this or add the option to the existing 3D text function.

Here's the example, in Sketchup Eight format.