Very interesting tool, although storing as typos does seem to be a bit visible and prone to mistaken 'correction'. Other approaches to consider might be:

* Changing punctuation for visually identical, but different characters. This would not work for printed documents however.

* Encoding only 'believable' typos, e.g. it's its. You could encode a binary stream across all instances of it(')s, or other substitutions.

* Encoding the stream in whitespace, e.g. Two/One spaces after a full stop. Printed documents would be lossy though (as full stops at line endings would be ambiguous). There are error detection/correction systems that can help though.