Hi,
personally I would use the "DEL" modifier (should have come with your keyboard :-) – I would find this well trained key stroke easier than remembering a new and arcane modifier (that only would work in BBEdit).
That said, you could achieve your goal by a workaround: create a Clipping that only contains your single quotation mark. That done, you can give it a key combination of your liking In the Clippings Palette (e.g. ctrl+alt+").
Of course you would have to create a seperate clipping for every "bracket glyph" – as said: a workaround.
Regards
Roland