So the end sky is kinda weird in vanilla MC. It has its own texture that is tiled several times along each side of a cube-style sky box. If you just put stars onto the texture, it becomes painfully obvious that he sky is a cube... and that the texture repeats on each face of the sky cube. So it's not like you can even just pull the normal skybox trick of distorting the texture with the curvature of a sphere to force it to look spherical when projected onto a cube.
The only reason that the end normally hides this is because it's effectively just noise which, when distorted at the edges and corners, just makes more noise. It was a quick-and-dirty way to get the effect they wanted at the expense of not being at all flexible. Still, it technically can be done.
And sadly I don't see anything in the Optifine docs that mention that its custom skies feature works in the End. I feel like it should, but I can't find anything to indicate that it does. Sorry.