Place structure on blocks underground?

[hexed hornet]

I have custom cave generation that I would like to place ancient cities in (picture provided). The terrain is generated as an inverse of overworld/sloped_cheese going from -192 to -16. I know villages and the like use project_start_to_heightmap, but I wasn't sure how to edit that or what other approach would let me place ancient cities relative to my custom sub-surface terrain.

Edit: I am on 1.21.1 Neoforge (latest) with Lithostitched and More Density Functions

[scenic spade]

This is not possible in vanilla

[hexed hornet]

drat. would you happen to know of a way to do it in modded or would I have to ask that somewhere else?

[scenic spade]

I do not

[hexed hornet]

Thank you, I appreaciate the help!