The only simple way (and it is hardly simple) to reverse cars on Magnum would be to do what SDC did with Thunderation: that is, reverse cars #3 and #4 of the 6-car train, as those are the only cars that do not have chain dogs or collision bumpers on them.
That said, reversing the cars would change the dynamics of the ride, potentially in an unhealthy way. Remember, the Magnum XL-200 train does not have headrests (thank God!) and thus whiplash becomes a concern with reversed trains. Magnum has not been tested with reversed trains or reversed cars, and reversing the train or cars is almost certainly in violation of the manufacturer's guidelines for safe operation of the ride.
In short, there are lots of reasons not to do it. Don't expect it to happen anytime soon.
--Dave Althoff, Jr.