From a pure theoretical standpoint, the speed limit would be whatever rolling speed can be achieved with max reverse thrust. If you deployed max reverse on a landing, and left the max reverse on until you stopped, then released the brakes and left it on as you started to roll backward, you would accelerate backwards, and if you were able to maintain steering control (unlikely) and the engines were able to operate with the turbulent reverse inlet conditions without stall/surge, you'd accelerate until there was no longer any surplus reverse thrust.
You'd eventually be careening along backwards along the runway at, maybe 30kt, 50kt, 80kt... who knows? You could probably estimate some number based on available thrust and rolling friction/air resistance, but safe to say it's never been tried, or even thought of. Don't even think of touching the brakes once rolling backward with any velocity.
From a practical standpoint, some operators may allow backing up under reverse thrust in specific circumstances, keeping in mind that you can't see where you're going so people need to be outside to clear the area behind, debris is being stirred up and ingested into the engines (ramp areas are a rich source of luggage zipper tags), and there's the problem of stopping without tipping over onto the tail when braking (the airplane's loaded C of G is not that far ahead of main gear, so it won't take much braking force for inertia to lever it over; safer to use forward thrust to stop than brakes).
So you may see power-back operations in the right circumstances, but the maximum speed in such cases won't generally be faster than a walking, (or maybe jogging) pace.