If you just want hide that geometry with a sky texture, then you should be able to do this with a cubemap (maybe like an interior cubemap, just with a regular sky + landscape), that is projected on a mesh, or your area, that should represent the sky. I guess, why you will not find much with sky portals, is because they are a special case, and a little bit more complicated, than your regular skybox. As far, as i understand, the difference is, that your sky portal is actually a texture, which is generated by a second camera in your scene, which sits in a tiny cube, isolated from the rest of the world, with your sky cube map, and additional geometry like some city buildings etc, to generate a more interesting skyline for the horizon. It was used in early DOOM and Quake and Wolfenstein games.
But nowadays, i guess you just need to project a good sky cubemap on those areas, that would solve the trouble from having two cameras.
Your real sky portal couldd be done too, with rendertargets, but afaik they are not exactly cheap in terms of processing power 
