use case for having vocab for TAG-ish vocab to represent content negotiation

Hiya

Expanding my comment from SWCG call:

I had a use case in FOAF to be able to represent in RDF that xyz was the 
concrete set of bytes we get by conneg as text/html, that abc was 
another set of bytes we get by conneg as application/rdf+xml, and that 
some other documents were the results of pgp signing one or the other of 
those...

http://www.w3.org/TR/Content-in-RDF/#base64ContentClass looks close, but 
i've not reviewed it yet

cheers

Dan