I have an idea for iconv that you can please forward upstream one day,
(as I recall upstream is not very friendly so I dare not forward it
myself.)

iconv [--replace-illegal-characters-with '<string>']

That way one could still get output, like -c, and at the same time be
aware of what was bad.

As to details, e.g., does it replace each char one by one, or a whole
string of them at once, I leave up to you.

Anyways, abcdef@@@@@abcdef looks good to me for output from
"iconv -c --replace-illegal-characters-with @" .