Thanks Adrian!
I decided to just tack ProcessWire's page ID on the end if the story ID is a duplicate:
public function changeName($event) {
$page = $event->arguments[0];
if ($page->template=='story' && $page->story_id!="") {
if (count($this->pages->find("story_id=" . $page->story_id))) {
// uh-oh, we found another page with this story ID
$page->name = $page->story_id . "-" . $page->id;
} else {
$page->name = $page->story_id;
}
}
}
statestreet's post in Using a different field to generate page URLs was marked as the answer