Aha, I finally got it working:
<?php
/*
Plugin Name: Akismet Better Privacy Notice
Description: Inform users about what's actually happening with their data
*/
add_filter (
'akismet_comment_form_privacy_notice_markup', 'better_display_comment_form_privacy_notice'
);
function better_display_comment_form_privacy_notice() {
echo sprintf(
'<p class="akismet_comment_form_privacy_notice">' .
'Warning: This site uses Akismet to filter spam.' .
' Until or unless I can find a suitable replacement anti-spam solution, this means that' .
' (per their <a href="%s" target="_blank">indemnification document</a>)' .
' <strong>all commenters\' IP addresses will be sent to Automattic, Inc.</strong>,' .
' who may choose to share such with 3rd parties.' .
'<br />If this is unacceptable to you,' .
' I highly recommend using an anonymous proxy or public Wi-Fi connection' .
' when commenting.' .
'</p>' ,
'https://akismet.com/privacy/'
);
}
?>