The challenge there is that the solution is convention rather than spec. Comments would only be understood by parsers that follow, or at least support, that convention and all other parsers would treat the data differently.

That may not be a huge problem for you, you see the "comment" key and know its just a comment and can ignore when a parser treats it like a normal string field. It could be an issue though, for example I could see any code that runs logic against all keys in the object becoming harder to maintain.