For those who can't read Jeff's response, here it is:

Richard wrote up the https://www.w3.org/community/schemabibex/wiki/Identifier, but I don't remember much discussion of it.

The abandoned "info" URI effort leaves me skeptical that non-HTTP URIs can be systematically described in general. I'm also skeptical that individual identifiers of any kind need to be described inline with instance data.

http://info-uri.info/

Jeff