I'm wondering if there isn't some way to have nebula as long as the nebula goes beyond the edge of the playing field. That way it could get denser as you move in one direction, and you could use the skybox to give the appearance of being outside the nebula, so the Nebula could get defineably thicker along one axis only, by letting the nebula poofs slowly increase in opacity as you move further away from the 'entrance' of the nebula and head further into it.
If you could get distance fogging to kick in the same way, using how far from the entrance you are, rather than how far from the centre, and use the right background, you could get a pretty good effect.
It wouldn't exactly be 'localised' nebula, but if you could define how quickly it got dense, and maybe have it go EMP at a predefined density etc, you could still tactically use it in combat situations.