DKIM for mail generated internally by PHP?

Some of my sites have private forums that let members subscribe to get email notifications when new posts/comments are created. The mails are generated by Drupal/PHP via either `sendmail -t -i` or `/var/qmail/bin/qmail-inject`. Obviously, neither of those go through qpsmtpd and the dkim plugin, and the mails go out without any DKIM signature and get refused by GMail and other major providers.

Is there a way to leverage qpsmtpd for locally PHP-generated mail, or do I have to provide a duplicate outgoing DKIM implementation in qmail?

Hans