I've actually heard of a turboprop going into beta in the air. They lost about 7000 feet but managed to keep it flying.
We don't have an established procedure, but we did discuss the scenario in systems.
In the Jetstream, you should only get a beta light on the ground. A beta light means that there is pressure at the beta switch, so the engine can go into reverse.
Basically, if you get a beta light in the air, you just want to make sure that you go full forward with the power and condition levers. Pulling them back could put the engine into reverse.