HRDP Stencil shader OR Depth Mask Culling [Insight Requested]

[I'm on Unity 2019.2, using HDRP 6.7.1]

Hello, I'm looking for insight on seeing through Terrain in a specific spot
(or rendering terrain invisible in a specific spot).

In default pipeline, there are stencil shaders & depth cull masking techniques.
But they appear not to work out of the box in HDRP.

I understand HDRP uses deferred rendering.
(Not sure what that means exactly,
but it sounds like it queues things up and renders them later).

But, if so, how do I take advantage of it to "see through terrain".

To give you a better visual of what I'm trying to do:

[Proc Gen a hole mesh, realtime.
This black sheet is the surface mesh, placed above the terrain with the intention of seeing through the terrain to reveal the rest of the mesh]



[This brown mesh below would be the rest of the mesh, though its supposed to be underground.
I want to use the black hole above as a depth mask to hide the terrain layer
and see the inside of the brown mesh
(which will normally be below terrain level).]



Traditionally, a "simple" depth mask shader seems like it might be the way to go. But since that doesn't work 1:1 in HDRP, surely there must be an HDRP version of this. Or some way to write a working mask for the TerrainLit Shader.

[If it helps, I've attached the TerrainLit shader]

p.s. plz don't make me write a volumetric shader for this >_>
(It defeats all the effort of writing the proc gen mesh script)

 

Did you ever solve this?

 

The next version of Unity came out with Terrain Holes so I didn't need to.
Shaders aren't yet my expertise, however - although I would still find this to be a very useful skill to write HDRP shaders.