Reverse is possible because of a planetary gearset aft of the chain. Going forward this gearset is locked up and has no ratio effect. In reverse this unlocks allowing the sun gear to spin in reverse. What this means though is that reverse has a different gear ratio than forward. I don't know what this ratio is (maybe 150%) but it's something to consider - basically it means there is less torque multiplication available in reverse. Also I'm not sure if the planetary gearset is durable enough for continuous use.
There are people running entire Audi transaxles in reverse. While a R&P does have drive/coast directions, the Subaru transaxle already "drives" off the "coast" side so I don't believe that's the issue. The R&P is only designed for the front drive wheels which will slip much easier than the rear wheels. If driving in reverse (or mounting the transaxle in reverse) the peak loads it sees will be much higher. In drag racing the front housing has been known to break on the 4EAT. This implies that the gears are strong enough, but the housing is not.
That said, I think as long as the torque is kept below the point that will distort/crack the housing, it should have a long service life.
Edited by presslab, 12 August 2013 - 12:15 PM.