Use Adobe Blank as fallback for all previewed fonts.
Mention font licenses in README.

Adobe Blank renders *any* unicode as empty and zero-width.
This way missing characters won't get previewed using a random system
fallback - every line will show glyphs from that font and nothing else.

This should work on iOS 4.2+ (Adobe Blank has no SVG version because it would be huge).
Should be harmless on <4.2 but I have no way to test.