Going to disturb the crickets here for a bit to update these two links:
The overall "authority" question (and related governance concerns) around non-URI link relationship values does need addressing.
@mnot has been in lengthy conversations on this front in the past and his input here would be most helpful.
That said, it may be best to break down the issues that need addressing here into related projects/tasks. There's a good bit in the initial post (governance, accessibility tech mapping, etc).
Lastly, it should certainly be noted that the use of rel values (thanks to the Link header and reuse of the concept/tool in other media types) far exceeds the scope of HTML5 and even "the browser." For instance, the popular Hypermedia Application Language uses rel values extensively--and it's among many similar peers in the space which do similar things.
to digging through the issues here and creating some achievable targets / goals.