Shoving binary into JSON is rather silly--base 64 encode it, or consider using a binary format better suited to your need.
As for adding comments to the spec--again, the idea is to prevent shadow information showing up that isn't obvious to a conformant parser.
If you give people the ability to smuggle data in comments, that's exactly what they'll do. At least this way people know that what they're doing isn't "to spec" and that their sneaky parsers are not following the rules.
I mean, JSON itself is rather silly in that it's a horrible serialization language. Its main boon is its C-style syntax, which is why it's so readable to all of us who read similar syntaxes all day. Even just a syntactical change noting which strings were base64 encoded would be nice.
As for adding comments to the spec--again, the idea is to prevent shadow information showing up that isn't obvious to a conformant parser.
If you give people the ability to smuggle data in comments, that's exactly what they'll do. At least this way people know that what they're doing isn't "to spec" and that their sneaky parsers are not following the rules.