Very interesting topic, thanks for having underlined this matter!
After reading further the explanations in that file, it should work, accordingly to the statement 'The "format" field serves a purpose to pass an information for your writer, so you may use the same writer for several output formats'.
What I haven't caught there is the correct procedure to achieve that, I'm afraid.... even it's more simple to understand and easy to make consequently, but I'm not an expert in making or editing .lua scripts.
With best regards.