Re: "wouldn't the charges involved modify the trajectory"
The only thing that can modify the trajectory of a photon is what is called gravitational lensing, and apparently you need a cluster of galaxies to obtain that effect.
So no, I don't think individual plasma ions are going to have an effect on the trajectory of photons.
Interestingly though, we are taught that light (ie photons) "bounce off" of objects, which is what allows us to see them. So you need entire galaxies to bend their trajectory, but a grain of sand can send the off in an entirely different direction. I still have trouble wrapping my head around that one.