Well, the cool thing about HTTP error codes is that you don't need a campaign or get permission from the W3C, you can just start using them if you want.
The issue is whether anyone else will expect this tag or code and do anything meaningful with it.
[0]: https://en.wikipedia.org/wiki/List_of_HTTP_status_codes
Tangentially to your point, that's only true of the 4.x and earlier versions of HTML which are SGML applications, WHATWG HTML / W3C HTML5 is not SGML-based and does not have a DTD.
You mean like this? https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/custo...