Oh yeah but I mean does a minehole ladder display a village territory influence then? What radius? If it doesn't, then this wouldn't really help as any facilities built down there wouldn't be claimed and you couldn't build a banner on non-village space down there.
I agree with the subject in general as I think you should be able to extend your village influence to different levels underground.
One thing with this though, is what about natural cave entrances in your village claim?