Thanks for taking the time to look at our specifications. The categories are dealt with in a separate IETF Internet-Draft (
http://tools.ietf.org/html/draft-johnston-http-category-header-00) but it's true that they could be layered on top of the Link: header. I hadn't considered this option at the time but it does make some sense. OTOH we need to sensibly layer properties/attributes on top of HTTP headers so we'll probably be blazing our own trail there anyway (using Property:/Attribute: headers modelled after Set-Cookie[2]:).