I think I was just lucky here
Here's what I managed after some trial and error - it seems to work:
in User_Email.php, find:
$m .= strip_tags($message);
replace with:
$m .= strip_tags($message). "\n";
$m .= $txt['user_email_answerto'] . $sec_email . "\n";
In Modifications.english.php, add:
$txt['user_email_answerto'] = 'To reply to this message please write to: ';
This is a partial solution to my problem, because if someone just hits reply it's still the admin that receives the email.