Hey Andy, not sure if it helps but here's how I've handled that before:

public class uDonutAttribute : DonutOutputCacheAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        if ( UmbracoContext.Current.InPreviewMode 
            || UmbracoContext.Current.HttpContext.Request.QueryString["dtgePreview"] == "1" 
            || UmbracoContext.Current.HttpContext.Request.QueryString["bustDonutCache"] == "true")
        {
            return;
        }

        base.OnActionExecuting(filterContext);
    }
}