If the author of this RFC is still doing edits, I think it might be worth mentioning in section 4.2 "Must-Ignore Policy" something along the lines of,

    An I-JSON implementation supporting a "Must-Ignore" policy SHOULD pass any such new protocol elements on, untouched, to any downstream consumers of the message, because those downstream consumers may understand the new elements.

It is another one of those things that are obvious to many people, but I could imagine somebody reading the last sentence of the section, "members whose names are unrecognized MUST be ignored", and thinking that they should omit the unrecognized elements before passing the message on.