The current examples used in the standard for idno are very numerically oriented. Would it be possible to add some non-numeric ones sure as:
<idno type="DOI">doi:10.1000/123</idno>
<idno type="URL">http://authority.nzetc.org/463/</idno>
http://www.tei-c.org/release/doc/tei-p5-doc/en/html/ref-idno.html
A typical WWP file has 2-4 <idno> elements. Two in the <publicationStmt> that refer to the file itself in formal terms. E.g., from our edition of Aphra Behn's _The Amorous Prince_:
<idno type="WWP">TR00241</idno>
<idno type="URL">http://www.wwp.brown.edu/texts/behn.amorous.html</idno>
These are always present. And then, 99% of files have 1 or 2 <idno>s in the <sourceDesc> to give the identification of the source document in somewhat formal terms, e.g. a library shelf number. E.g., from our edition of Joanna Cartwright's _The Petition of the Jews_:
<idno type="LT">Thomason Tract E.537(17)</idno>
<idno type="Wing">C695</idno>
Hope this helps.
At 5815 I've added some of these examples to the <idno> spec, pending a more thorough revision of its discussion in the body of COBI.
I've changed the DOI example to match the original proposal: the revision at 5815 was obviously too hasty.