probably not. no physical sky texture ever does in any other renderer.clouds can be dealt with compositng or vdb files or geometry with sky shading.
You can do that with OSL, I have a preview of something I am working on, not released yet here.
I also have several other constructs going doing other types of ( note: Realtime clouds ) - So it wont take any effort to render with Arnold.
Same principle. Just replace the cloud shader with a noise shader.
Environment mapped clouds are way faster to render than true volumetric clouds. If you don't need "god rays" or volumetric shadows, environment mapping is the way to go. VUE has a great rendering engine for environment mapped clouds. There's no reason why that can't exist in Arnold or any other application. It's just not a trivial thing to ask for, and I don't think the Arnold devs are going to devote any time to it, ever.