Currently, and as always, Endnote is blind between male and female authors. It only gives the option of id., not ead., for repeated authors in footnotes, for example. One solution would be to allow tagging of first authors by gender, and then keying repeated citations to their gender. “Id.” before a female author looks ridiculous, and it does happen at least 50% of the time, after all!
(I won’t speculate on the gender of the EndNote programmers. 
)
Are there many style guides that still follow this antique practice? I know that Ritter recommends it in his “Oxford Guide to Style” (2002), but even he concedes that it is often impossible to identify the gender of a cited author.
As a librarian, I occasionally get inquiries from students who are trying to locate a journal with the title “Idem”. Interestingly, I’ve never been asked for a journal called “Eadem” or “Eidem” or “Eaedem”.
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.