I finally found how to reproduce Firestorm's Windows build behaviour: they simply list more Windows fonts as fallback fonts in fonts.xml... Here is the result for next release:
As a result I also removed the "extended character set" settings/hacks from next release (since they are not needed any more for Windows and won't change a thing under Linux, where automatic fallback happens with system fonts and fontconfig/freetype; sadly, Linux won't display such silly characters that only exist in M$ fonts...).
This said, that NGW scripter still deserves being shot in the head for
utter stupidity (all this
waste of time for fancy characters that don't display in all viewers,
not even the official SL one, and serve no purpose whatsoever !)...