Combining Stencil and worldspace "reverse" clipping?

I've built a working surface shader (call it "wonderland") that renders as invisible unless a companion "lookingGlass" shader intersects with it from the viewpoint of the camera. Simple stencil shader arrangement.

Easy peasy.

I can add shader settings to specify a plane, or even just a minimum worldspace Z value, and use clip() to only render pixels on one side of that plane... (in other words, I could use that to trim the content that's allowed by the Stencil.)

What I want to do is use the stencil on surfaces "through the looking glass", (to reveal geometry that's inside the looking glass) and to always render those surfaces when they're on "our" side of the looking glass (to always show them if they're on this side of the looking glass portal). eg., if z<0, render if the Stencil Ref value is satisfied. if z>=0, render regardless.

Now, in Unity I *can* attach two materials to the MeshRenderer component (one with a stencil shader, one with a "plane cutoff" shader) - that works fine. It's pretty awesome, actually, at least visually. But while I haven't benchmarked it yet, I instinctively believe it's going to massively impact framerate if there are a number of objects, fairly complicated geometry, etc., set up with this arrangement.

(I can also manage shader attachment in code, and only do this when I expect something to transition, but I'm really hoping to get a unified shader out of this to avoid unnecessary draw calls.)