It looks like ITU-R has taken over International Morse Code and it's now
specified in ITU-R Recommmendation M.1677-1 (10/2009):
The footnote on page 1 claims that the definition of morse code in ITU-T
Recommendation F.1 has been withdrawn, which would leave M.1677-1 as the
best official definition of International Morse Code.
M.1677-1 includes a code for '@', which I hope is definitive enough for
the maintainer to add it to morse(6). Of course the manual page will need
to be updated to refer to the new standard.