the only problem with circular uvmaps may be at the two poles.
personally I would use it if I'd have to use a single texture
you may want to use it even if you use a tiled starfield as background, before applying an image over a specific area.
The specific area could be mapped with planar (expecially if small or at the poles) or using a portion of the same circular uvmap.
The scaling problem should be not too hard to avoid if you use lithunwrap:
to set-up the tileing in Lith you just have to make the uvmap bigger than the map, the problem is to decide how much bigger.
If I have, example, a starfield of 400x300, and the stars are at the same scale of a 800x600 photo that I'll use for a specific area, the uvspace for the tileing should be (proportionally to an equivalent area) double than the space used by the 8x600 image.
And to be sure of the result, you can use the preview window and adjust the UVs until you obtain the desired result.
There is a strategy that you may find very useful:
create a new image, fill with white, draw circles filled with different colors at the 4 verts of the image, and one in the center; and maybe draw the 2 diagonals to connect the 4 verts.
Then you can use this test map to verify that you are uvmapping correctly (the colored circles must be not distorted and in the desired position).
In my example, I could create a 8x600 test image and then a second 4x300 image with the circles of the same size of the one before. If my mapping will be correct, the coloured circles will all be of the same size, then I'll have to just substitute the test maps with the real ones
