I don't know if their new deferred path will make it into 4.x. That said, deferred support for SF is almost done :)
(In fact, it's done, though I accidentally broke some things in the forward rendering path's lightmapping system)

This has been discussed a lot back and forth, and there are many things to consider:

http://forum.unity3d.com/threads/222049-Shader-For...

Plus a lot of discussion on it on this page:

http://forum.unity3d.com/threads/222049-Shader-For...

So, as I mentioned, I may open it up at some point, but we'll see when/if it happens.

There's no way of accessing the background normals yet.
You can't really do replacement shaders or post FX shaders in SF in a nice way at the moment, but it is possible, by attaching planes in front of the camera, but I believe it will most likely lead to visual artifacts and maybe sorting issues as well

Right, this is because cubemaps need to be readable in order for Shader Forge to render them. Go to the cubemap import setting and make it readable to temporarily avoid this error :)

Lightmapping is probably one of the more time-consuming aspects of SF, because it's very undocumented, and Unity's shader code is adapted to fit only for surface shaders, and require quite a lot of reverse engineering to implement them in the CG shaders that SF is using.

I'm currently working on implementing deferred rendering, which also of course has lightmapping in it, and I'll take another look at the forward rendering lightmapping as well :)

I don't think you can read from the skybox automatically at the moment. However, as soon as Marmoset release their next Skyshop update, you'll be able to read from it easily :)
At the moment you have to do it manually.

Multiply the alpha by the RGB value, and it should work - at least when using additive rendering

It's because you were using vertical | pipe | characters | in your property name, which breaks the serialization. I removed them in the shader here:

http://pastebin.com/GGJYCX85