Are there any safety features to prevent reversible-pitch props from reversing in flight?
Beta props are pretty rare on anything smaller than Twin Otter or King Air or Porter or Turbo Beaver.  Can't speak for MT's system, but most beta mode systems use mechanisms to switch control of propeller blade angle from the propeller rpm governor to the throttle.  Beta mode is the mode of throttle or power lever control of blade angle, with engine torque controlled by a governor system of some kind to regulate rpm.  

Beta mode is very similar to turbine helicopter collective control, where a lever, the collective, directly changes rotor pitch and a governor system regulates rotor rpm by adjusting engine torque.

Beta mode is normally entered just above flight idle (flight beta) once the propeller speed governor is no longer regulating blade angle (blades at fine pitch stops).  

There is normally some kind of detent or gate that requires a deliberate movement or unlatching to move below flight idle (the Twin Otter requires you to twist the throttle grips - on others there are finger triggers).  

Also, some turboprops have beta lockout systems that prevent power lever movement below flight idle until you're on the ground.  But a lot of airplanes don't and can be put into ground beta in flight if the flight idle gate or latch is deliberately operated by the pilot.

During the Twin Otter program there was some controversy at DeHavilland Canada because demo pilots were using ground beta (coming into DISC - blades flat, like a disc) to make crazy steep descents (I've also heard of Porter pilots doing it).  A very dangerous thing to do, because if the prop doesn't want to come out of beta mode when you move the lever(s) back up, you are dead meat.