You could add a copy of the preloader to the app background.
How do you mean?
var preloaderCopy:CMSPreloader = new CMSPreloader;
where staticBackground is a UIComponent and CMSPreloader extends SparkDownloadProgressBar
It is not possible to leave the preloader in the background? What happens when the complete event is dispatched, it gets removed where?
I'd just mimic the artwork. If you want it to actually work, consider using
an mx:ProgressBar to replace the preloader.