Actually, this probably wouldn't be that big of a challenge. Stars each have their own skyboxes that fade as you zoom away from them. I don't know if you could do anything about the zoom-fade, but technically speaking, for flying around inside it, you would just need to create that skybox and attach it to a star, either as an optional skybox, or create a star specifically for it. I'd probably suggest the latter so that when you do zoom out far enough to see a galactic view, you can have a dyson's sphere icon replace the star icon. This could also be done with a ring-world in the same fashion, but making star icon with part of the ring 'blocking' light from it.
As for the planets or no planets part.. well, the first is easy, use planets. For no planets inside, the easiest way to go about that would be to create custom planets, but use, say just for ease, the orbital buildings for any given race as the mesh. I may suggest resizing them to be a bit more appropriate as massive structures that'd actually house the people and pump out the fleets that'd be created in a normal game, though.
'Course, both parts of the above have already been mentioned, I'm just more or less confirming that that's pretty much all there would be too it, discounting any fully custom stuff that would be wanted beyond textures/icon images. But, before anyone suggests I do it, since I'm kinda making this sound easy, I'm sadly *not* a texture artist (and how I seriously wish I was.. Or all that artsy at all...), or I honestly would just take up the project, because it'd be a fun mod (or mod addition) to have.
