This thread on TEI-L:
http://listserv.brown.edu/archives/cgi-bin/wa?A2=ind1204&L=TEI-L&F=&S=&P=57936
demonstrates that guidance on linking quotations with references should be improved, probably in the Core chapter, 3.3.3. Assigned by the Council FTF to MDH, September 2012. This arises out of the now-closed ticket http://purl.org/TEI/BUGS/3520414.
Sorry I seem to have missed a lot of this discussion (SF doesnt seem to prompt me about updates any more) ... but for the record, I agree that this (definition of a new attribute @source) is a very good solution.
Last batch of changes submitted in rev 11387; once they've gone through, we should have a good look at the changes. There's the bit in the CO Chapter (in COHQQ), the new att.sourced (with two examples), an addition to the bibliography in support of an example, and the original bit in the TS chapter relating to <writing> and @source.
Changes have percolated through Jinks. Please take a look at the new attribute class:
http://teijenkins.hcmc.uvic.ca/job/TEIP5/lastSuccessfulBuild/artifact/release/doc/tei-p5-doc/en/html/ref-att.source.html
and the chapter section:
http://teijenkins.hcmc.uvic.ca/job/TEIP5/lastSuccessfulBuild/artifact/release/doc/tei-p5-doc/en/html/CO.html#COHQQ
with new text towards the end (example from Tolkien).
This was completed in January. Closing the ticket.