That's some out of the box thinking! IANAPP (I am not a plasma physicist, although I have a bachelor's degree in physics), but I thought I'd mention a few things to think about
:
1) It takes some power to generate the ions/plasma and disperse it. Generally, this will scale with the amount of current you need to get enough ions in the air to make it react as you want. I don't think the room ionizers use much power, but moving down the road means you are trying to inject ions into a very large volume, so the power required is significantly more.
2) Generating a stong magnetic field electrically takes a lot of power (--> high load on your alterntor), but perhaps a configuration of permanent magnets could be used to guide the plasma flow. However, permanent magnets would not be easily tuned for different vehicle speeds.
3) Static charges need to be properly dissapated so they don't build up and cause a safety issue or equipment failure.
4) Plasma wind might block your radio reception (although I'm guessing that the level of ionization contemplated here would not affect it too much)
5) Plasmas can be induced to emit sound... imagine a vehicle-size subwoofer!
