For anyone who sees this in future:

I solved this by adding an event handler which fires on page requests:

    public class ApplicationEventHandlerOverride : ApplicationEventHandler
{
    protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
    {
        PublishedContentRequest.Prepared += PublishedContentRequest_Prepared;
        base.ApplicationStarted(umbracoApplication, applicationContext);
    }

    private void PublishedContentRequest_Prepared(object sender, System.EventArgs e)
    {
        var request = sender as PublishedContentRequest;
        if (request == null || request.TemplateAlias != MyEnums.EnumX.ToString())
            return;

        var ancestor = request.PublishedContent.AncestorOrSelf(3);

        request.SetRedirect(ancestor.Url);
    }
}

Source of my answer was found at: https://our.umbraco.org/forum/developers/api-questions/47762-redirect-all-nodes-with-no-template-to-ancestor-with-a-template-