Why not use a set of very distinctive fonts, a different one for each format, and write down which you used for which? Or is there some other constraint you haven't mentioned?
Or.... why not just use a regular glyph position, and replace the content? Take a common character (say, numeral 1) and replace it with a glyph that displays the name of the format. So the EOT font has a glyph that says "EOT" and so on. Start with a font that has a license that permits this, probably an open source thingie.
Cheers,
T