I have looked at mcexplore and pymclevel, and they both appear to only support vanilla maps, in addition to only supporting setting the location of the player rather than loading specific chunks. I am wondering if there is a concise way to do this from the command line that supports cubic chunks. If I have to make a mod, then let me know.

Mcexplore should work. It won't allow specifying Y range but can be modified to do it.

Aside of that CC config allows to specify generation range for spawn area. So it can generate more chunks at a time.

Making gorge pregen command work is also planned.