How about the "tag" URI? draft-kindberg-tag-uri-02.txt
This document describes the "tag" Uniform Resource Identifier (URI)
scheme and the "tag" Uniform Resource Name (URN) namespace, for
identifiers that are unique across space and time. Tag URIs (also
known as "tags") are distinct from most other URIs in that they are
intended for uses that are independent of any particular method for
resource location or name resolution. A tag URI may be used purely as
an entity identifier.
Bill