Adding a custom field from content_save_pre filter through XMLRPC

Is it possible to add a custom field from a content_save_pre filter? I’m trying to detect some fields being typed in the post content, e.g. [something]Thing[/something], and then set a custom field to “Thing”.

The following code works fine when posting through the web interface, but fails on the clients I’ve tried (MarsEdit and WordPress iOS) — the [] tags get removed, but the custom field doesn’t get set.

function dfll_get_link($post_content) {
  $dflink = dfll_find_link($post_content);
  if ($dflink) {
    global $post;
    $post_id = $post->ID;
    update_post_meta($post_id, 'linked_list_url', $dflink);
  }
  $temp = '/(' . dfll_regesc('[ll]') . '(.*?)' . dfll_regesc('[/ll]') . ')/i';
  $post_content = (preg_replace($temp, '', $post_content));
  return $post_content;
}

add_filter('content_save_pre', 'dfll_get_link');

(Assume the other functions are defined.)

This is for my Daring Fireball-style Linked List plugin. Code above adapted from Justin Blanton’s Slugger+ plugin.