The mainland is cluttered with skyboxes, and private region owners may be miffed to discover that putting something at 1000 m is no guarantee that it won't be visible from the ground by someone with a very high draw distance. A possible solution: culling surfaces.
Whenever the viewer is asked to render a pixel with TEXTURE_SKY assigned to it, it should simply render the sky buffer instead of showing anything behind that surface. (This would also be handy for making magic portal effects.)
For bonus points, implement this as a prim flag (like reflection probes) that actually causes anything inside it to be forcibly culled from the outside, unless the camera is inside a linked culling prim. Implementing this detection test doesn't need to be very sophisticated; nearly all use cases are covered by scaling, translating, and rotating cubes.
A creator-centric solution to these two (very similar, possibly should be merged) requests for blocking the display of skyboxes: