Set a dynamic default property when a page is created?

Question

Is there a way to set a default property that is dynamic for a page when it is created? For example, i want to capture a user id from our system and store it in an "author" property when the page is first created. This value can be changed to a different user by the author but in case they don't set it i want to have it default on page creation.
I guess a better question would be, is there somewhere i can put code so that it will be executed when a page is created?
I considered writing a custom widget and putting this in page properties, but that won't actually run unless page properties has been opened.
Thanks

Answer 1

For anyone in the future, here is how i implemented the event handler -
@Component
public class PageCreationObservation implements EventListener {
    Logger log = LoggerFactory.getLogger(this.getClass());
    private Session adminSession;
    @Reference
    SlingRepository repository;
    @Activate
    public void activate(ComponentContext context) throws Exception {
        log.info("Activating PageCreationObservation");
        try {
            String[] nodetypes = {"cq:Page"};
            adminSession = repository.loginAdministrative(null);
            adminSession.getWorkspace().getObservationManager().addEventListener(
                this, //handler
                Event.NODE_ADDED, //binary combination of event types
                "/content/appname", //path
                true, //is Deep?
                null, //uuids filter
                nodetypes, //nodetypes filter
                false
        } catch (RepositoryException e) {
            log.error("Unable to register session",e);
            throw new Exception(e);
    @Deactivate
    public void deactivate(){
        if (adminSession != null) {
            adminSession.logout();
    public void onEvent(EventIterator eventIterator) {
        try {
            while (eventIterator.hasNext()) {
                Event newEvent = eventIterator.nextEvent();
                log.info("something has been added : {}", newEvent.getPath());
                //Check if node exists and it is a content node of the newly created page
                if (adminSession.nodeExists(newEvent.getPath())
                        && adminSession.getNode(newEvent.getPath()).getProperty("jcr:primaryType").getString().equals("cq:PageContent")) {
                    Node contentNode = adminSession.getNode(newEvent.getPath());
                    if (contentNode.getProperty("jcr:createdBy") != null) {
                        contentNode.setProperty("author", contentNode.getProperty("jcr:createdBy").getString());
            adminSession.save();
        } catch(Exception e){
            log.error("Error while treating page creation events",e);