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is a global configuration, and I don't believe I can say for a given font,
"these glyphs are intentionally blank", which leaves me with these two
questions:

1. Is my understanding correct, that I can't override <blank> per font?

2. The fonts.conf page says that 'fonts often include "broken" glyphs which
   appear in the encoding but are drawn as blanks on the screen.' A trawl of
   the mailing list suggests that this configuration option is ancient -- at
   least I can't find any discussion of its introduction -- so is this form
   of breakage common in fonts we use today?

I am aware that I'm attempting to do something rather odd here, so I think
I'll have to do my own substituting with spaces when producing "screen
shots" of these old terminal displays, but I wanted to check my
understanding of fontconfig's blank mechanism.