I drove the McCarthy road in a 1984 Toyota Tercel with no issues. On the other hand, my old boss and a buddy drove the same road in lifted off-road 4WD trucks and BOTH trucks got railroad spikes (the road follows the course of the old railroad bed) impaled through their tires (fortunately, they were both carrying full-size replacement spares). Hopefully with ongoing improvements and maintenance, those risks have been minimized, though.
I know from experience that the cars get filthy inside and out when driving on such unpaved roads, so it is pretty difficult to hide the fact you have driven on such roads. If you don't clean the car yourself, you will probably be hit with a cleaning fee.
Cleaning fees are pretty much never charged in ANC. Cars get dirty in lots of places, not just on the restricted roads. The only time cleaning fees are charged is if you return the car with spilled fish guts in the trunk (or, more commonly, in the back cargo area of a minivan--fishermen love minivans), as the car is pretty much out of service for two weeks while the rental agency tries (mostly in vain) to rid the car of fish odor. Cleaning fees are never assessed for simple mud/dirt--the dirtiest cars that get returned might take 30 minutes to clean (versus 5-10 for a normal return), but that's not nearly enough to assess any penalties against the renter.