Currently the angle the sky is viewed from isn't affected by sector rotation (as in Sector_SetRotation or the UDMF rotation fields).
Rotating the sky angle according to the sector rotation would be useful for getting a panoramic sky texture lined up properly across a portal.
Also, it would be useful if that sector rotation angle were applied to the viewing angle of skybox cameras as well, so that setting the proper rotation on a sector would also fix skybox rendering in the same situation.