If Wendell says it can't be done I'd say it can't be done. I think instead of removing the scratch you'd have to fill it first with something then polish the filler. Trying to grind it out or polish it out would only compromise the glass and give you a weak spot.
You could try something like speedy auto glass, try to find out what they use to repair car windshields but it may not be reef safe or suitable for underwater use.
I've tried using two part epoxy on glass and found it didn't stick, water got behind it and it lifted off so don't waste your money on that.
Doug
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