Achieving a multi pass effect with a Surface Shader

Heya! I've been scratching my head over this and was hoping someone might be able to point me in the right direction.

I'm trying to add a silhouette effect to some characters so they're visible through objects, this is a mockup of what I'm going for :



Normally I'd do this with a flat shaded second pass which uses "ztest Greater" but I'm using a Surface Shader which...

Achieving a multi pass effect with a Surface Shader