Stencil buffer custom usage with deferred lighting?

I need to use stencil buffer for some simple clipping (front faces that passed ztest increase stencil, back faces that passed ztest decrease it)...

Setting stencil:

 Tags { "RenderType"="Opaque" "Queue"="Geometry+10"}
 ColorMask 0
 ZTest LEqual
 ZWrite off
 Lighting off

 Pass {
     Cull Back
     Stencil {
         Pass IncrSat
     }
 
     CGPROGRAM
     #pragma vertex vert
     #pragma fragment frag
     ENDCG
 }

 Pass {
     Cull Front
     Stencil {
         Pass DecrSat
     }
     
     CGPROGRAM
     #pragma vertex vert
     #pragma fragment frag
     ENDCG
 }

using it later:

 Tags { "RenderType" = "Opaque" "Queue"="Geometry+12" }
 Stencil {
     Ref 1
     Comp GEqual
     Pass Keep
 }

However it looks like it's not really working with deferred (it only works with Ref 1, Pass replace, which is unusable because I can have overlapping areas that need to set stencil).

What mask can I use in deferred, what bits are not used/cleared, etc.?

Every example I can find is probably only for Forward... Documentation (link text) is very vague at best.

Any help/insight appreciated!