Melt the glass. Take a blowtorch to it and you might be able to knock the sharp edges off of the scratch. I cannot imagine that this would work well at all in practice. You're far too likely to melt soot into the glass, have a nasty sag, or torch a hole through it, or all of the above, assuming you can get it to melt at all. Sounds like a fun thing to try after you've already replaced the glass, though.