Ok, I've got an editor worked up. I did it in Excel to take advantage of some formatting and editing conveniences. It gives a visualization of each glyph from the Adafruit_GFX lib 5x7 font, and lets you edit and get revised output to a custom version of the font file (glcdfont.c).
There are 2 versions in the zip: GlyphEditor_5x7.xlsx: This is the best one, uses Excel conditional formatting rules for a nicer matrix visualization. May not work in other spreadsheet editing programs. GlyphEditor_5x7-textonly.xlsx: If the one above just shows ones and zeroes in the matrix viz in your spreadsheeting program, use this one instead. It does not use conditional formatting.