I am asking for a future-save file handle that allows package authors to
write to the standard \jobname.aux file, using \iow_* functions from the
l3file package.

I guess that many L2e package authors use *.aux in order to avoid
inventing private aux-file endings which may already be in use and
which would give rise to conflict. Moreover, reading in \jobname.aux
\AtBeginDocument is already been taken care of.

Another idea would be to implement dedicated functions to write to
\jobname.aux in analogy to the already existing ones in l3file.

Kind regards,