Instead of allowing special characters in some fields and not others add a literal escape so users can output : or [ in fields. See http://docs.oracle.com/javase/1.5.0/docs/tooldocs/windows/javadoc.html#%7B@literal%7D

Use case:
I want to set the fieldname of my GET parameter to aaab[ccdd] but the parser only recognises alphanumeric, slash etc. Allow me to manually escape the [ and ] characters.