I really is crucial to come to a complete halt when changing from forward to reverse. I pick up the clutch in neutral and put it straight back down again thus avoiding the usual crunching & grinding.
I've done this in cars when I've had to drive a short distance with no brakes. It is possible at very low speed but the noise & feel of it makes me think I'm definitely causing major damage.
I don't think it's possible in most cars, there is a physical mechanism to prevent this from happending which causing the cogs to clatter over each other rather than engage.
I guess it depends how the car is setup but I doubt a gearbox could survive a reverse engagement whilst going forwards.