This is a few years' late, but for those still searching on this - here's the answer from a Moodle 2.4 user:
If you look in the certificate.php file under the Type folder and go down to the print strings, you'll notice there is a 'C' before the font family.
If you want to move the element to the Left, change the C to an L and type in your settings. If you want to move the element to the Right, change the C to an R. You may wish to replace the $x and $y with set values.
certificate_print_text($pdf, $x + 112, $y + 112, 'C', 'freesans', '', 20, certificate_get_date($certificate, $certrecord, $course));
Many thanks to Paul Johnson of Klevar for helping me work this out.
Online Learning Specialist
Australian Institute of Social Relations