I managed to get it working by adding this <script type="text/javascript" src="https://apis.google.com/js/plusone.js"></script> to the header of theme
and then adding $visible .= '<li class="share-Google"><g:plusone count="false"></g:plusone></li>'; to line 421 in file jetpack/modules/sharedaddy/sharing-service.php and it seems to work fine.
eg.
Line419 // Visible items
Line420 $visible = '';
Line421 $visible .= '<li class="share-Google"><g:plusone count="false"></g:plusone></li>';
Line422 foreach ( $enabled['visible'] AS $id => $service ) {
Line423 // Individual HTML for sharing service
Line424 $visible .= '<li class="share-'.$service->get_class().' share-regular">';
