Cylindrical cubemap can't be compressed? Workarounds?

I have purchased the AllSky asset from the asset store and it includes some great-looking skyboxes, which come all in the format of a 3072 × 1536 PNG file and the import settings are: Texture Type = "Reflection", Mapping = "Cylindrical"

According to this: http://forum.unity3d.com/threads/165125-Has-the-CubeMap-texture-compression-bug-been-fixed-in-Unity4 it looks like cube maps can't be compressed. I have looked in the editor log, and indeed, no matter what compression setting i choose, it won't PVRTC-compress the textures when building for iOS. The 16-bit compression works, but that's not what I want, the texture is still far too big with 16-bit compression.

Maybe PVRTC can't compress 3072 × 1536 sized textures. But I really need that skyboxes to be compressed. Right now, one of those skyboxes is taking 6.5 MB, with the 16-bit compression still more than 4 MB.

My question is: If it really is not possible to compress such a cylindrical reflection cube map, Can I convert the cylindrical skybox anyhow to a 6-faced skybox with separate images for front, back, left, right, top, bottom? If I could convert it to distinct images, I could PVRTC compress each of that separate images, because I know, that is working.

How do I do it? Which tools can I use? Or is there a way to compress it?