Hi Tom

You are looking for PublishingStrategy

public class Register : ApplicationEventHandler
{
    protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
    {
        Umbraco.Core.Publishing.PublishingStrategy.Publishing += PublishingStrategy_Publishing;
    }

    private void PublishingStrategy_Publishing(Umbraco.Core.Publishing.IPublishingStrategy sender, Umbraco.Core.Events.PublishEventArgs<IContent> e)
    {
        foreach (var content in e.PublishedEntities.Where(x => x.ContentType.Alias == "MyDocTypeAlias"))
        {
            content.SetValue("MyPropertyAlias", "My New Value Everytime Someone Presses Publish");
        }
    }
}

Cheers