Would a seperate box not have difficulty in seperating road noise from actual audio (unless it had some level of DSP function built in to differentiate).

It would be no different than the mic input on the empeg itself in that respect. You'd have the same problem.

My thinking was that the only way to get around this problem would be mic placement. It would need to be placed outside the cabin somewhere, such as under the car or in a wheel well. I'd say to put it in the engine compartment, but on my car I get more variation in cabin noise level because of tire noise than I do engine noise.