It doesn't look like there is a way to accomplish what you're asking for with the current version of DAM. Hopefully, a future release will contain more options for organization. However, I don't have any info on when or if that might happen.
Anyways, thanks for the info... I have checked AEM assets and I think can use the "reference tool" to check the files one by one if they are being used on a page (its not a quick way but at least I can start on this) Regards