I'm not seeing how that relates to what $encode and $unsafe do. The closest to such identifier I'm aware of is how /drawsave chooses the filename when you give it a filename that contains characters it doesn't like. Plus, changing to underscores causes too many inputs having identical outputs. Since upper/lower case alphabet letters are treated as equivalent for filename purposes, there aren't even 64 characters from the printable 33-126 range that can be used for filename characters.
