> Output filtering has the complimentary problem -- you need to understand every possible input.

No, you simply need to understand the encoding rules of the sink. Which is precisely why "sanitizing input" is plain nonsense: Whether a particular unescaped character has some meta character function is not a property of the character, but of the output language, so you can not possibly "sanitize input" in any meaningful sense, unless you mean by that "randomly garble the input".