You can back up in a B-1900, but there is a time limitation for being in reverse (or beta for that matter) to prevent overheating of the oil cooler. It must be designed to have some airflow though it on the ground.
I've also seen Northwest powerback DC-9's from the gate in MSP. (Been on one three times in fact). Kind of neat. First they check if they have normal above idle forward thrust (stopping with brakes while backing is not recommended!) Then they open the clamshells and giver!
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