i cannot say much about the dhc8 but in our cheyenne III its definitivly prohibited. when you see the amount of speed loss pulling flight idle with props forward you can imagine what happens when you go into ground idle /flat pitch or even reverse being airborne.
on the pt6a ground idle is not governor controlled so a prop "runaway" might occour doing it airborne. the enormous drag could result in a loss of control with catastrophic results.
but thats just a guess- i never tried it and i never saw it , i think you will find little pilots which can tell you from real experience what would happen. i heard some stories where guys after deploying parachute jumpers dived back to the field in ground idle on a caravan , but cannot confirm if its true.