Using added text characters

If you're talking about the "big font" (dialogues), characters 0x80 to 0xFF are use for the DTE table, where one byte equals two characters. So if you're using empty characters it must be between characters 0x20 and 0x7F (there's one used for space but I don't recall which one exactly). I don't recall if there are similar restrictions for the small font, the one used for menus.