just checked Chicago Manual (online) and found only “idem”, but also what seems like a warning against using it altogether. In a query discussion outside the manual proper the answer given was just like yours. So perhaps that is their view after all. I wonder if idem is masc. standing for fem. (as can occur in Latin) or whether it is neuter (ie long or short i)… Definitely not advised for plurals though.
Thanks, John
Here’s CHicago 15
16.49“Idem”
When several works by the same person are cited successively in the same note, idem (“the same,” sometimes abbreviated to id.), may be used in place of the author’s name. Except in legal references, where the abbreviation id. is used in place of ibid., the term is rarely used nowadays. It is safer to repeat the author’s last name.