Ah. Well, caves are hardcoded mapgen. Affecting them would require c++ or someone to jsonize them, both of which are beyond my personal abilities. Unless I’m misreading the code, and I hope someone corrects me if I’m wrong.
I don’t think they’re opposed to adding code for mod support, so that’s probably not out of the question, but you probably just want to work inside of the json?
I’d suggest making your own location from scratch rather than trying to attach it to the existing caves. It wouldn’t be hard to draw up a surface location that would reflect the cave entrance, and throw an underground for testing. In the end, you could match the symbol and name and all, so players wouldn’t know if it was a hardcoded cave or yours until they went inside. If that’s what you’d want.
If you’re really focused on testing the underground, you could just copy the LMOE shelter surface floor. It comes with stairs already and is pretty minimal. That’s what I did with the caches mod. Alternatively, spawning the location with the debug menu will work even on the surface of the world, it’ll fill in the rock and everything, so you really don’t need to be underground for testing.