[tei-council] add idno to att.canonical?

[Lou Burnard]

<idno> was fairly recently added to content models of <person> <place> 
etc, so as to permit inclusion of data from various authority files e.g. 
VIAF, ISNI etc. But some of these authority file records also contain a 
preferred URI which cannot be determined automatically from the 
authority record's identifier. It would be useful to use @ref (or even 
@key) on <idno> for this purpose.

Before I add this as a FR, does anyone on Council have a better suggestion?