I would think that noise cancellation would be better done with a hardware controller somewhere closer to the amp.
As in, a processor that has its own mic input, and is programmed just to take the inverse of that mic input, and apply it to input, then output the result.
Doing it on the computer and having to do it in real time would probably be more prohibitive.
However, doing it in the last stage before the amp would be relatively easy I would think? And you could even have the mic on a lead, so you could place it up near the headrests of the seats, so as to be the most effective.