from the set $char(1)..$char(31)
Mp3tag shows improbable glyphs ...
... or $char(9216) ␀
... or $char(9248) ␠
... or $char(9250) ␢
and hollow boxes...
... or $char(9251) ␣
but there shows ␣ , so I think that's the safest. It tests OK on WMA, but I don't know about other formats.
BTW, for anyone like me looking for tricks to format a Creative ZEN display: all those characters are successful in affecting sort order, but appear as hollow block, rather than invisible. If anyone finds a sort-affecting character that is invisible, please do tell!