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.)
As it turns out, what I was looking to do is impossible.
The two shaders I wish to combine are both surface shaders. While you can combine multiple surface shaders into a multipass shader, you cannot combine multiple surface shaders, with a Stencil, and with a clip() where the clip is applied to passes that the Stencil is not and vice-versa.
There are combinations that can achieve parts of this, or can achieve the entire goal with surface and vert (or other non-surf) shaders, but the combination of requirements stipulated by this question isn't supported as desired.
While this does not answer the question, the workaround in Unity is to create two materials that provide each piece of functionality. They can both exist on the item that needs both pieces, and code can otherwise manage whether one or the other or both is actively in use.
Similar solutions would be available in other packages.