I have tested it in many 3d apps but haven't tried it in RF but its is realy simple.
It involes two simple models.
Model 1 is the base modle for your character.
modle two is the black line drawing modle.
Modle 1 is slightly smaller than modle 2 (All verts need to be inside modle2)
Make modle 1 fullbright. Make modle 2 colored jetblack. Then place modle2 right on top of modle one and flip the normals.
You'll get this


Simple modle so the linse are very corse but it can handle any complexity if RF can handle it.