Any idea if SharePoint has an extensibility point to add an external DAM store as a normal Asset source for images or other media files?
OOTB SharePoint hasn't this feature, btw you could use the RBS Implementation: http://technet.microsoft.com/en-us/library/ee748649.aspx
or a solution from Avepoint or Metalogix
For Videos at least you could consider leveraging IIS Media Services. Microsoft has a few posts on leveraging both on site and Azure media services. This doesn't require RBS, however the video source will not actually be SharePoint.
Do a search for:
"Delivering End-End Video Workflow Using Microsoft SharePoint,IIS Media Services,Microsoft Expression Encoder and Microsoft Silverlight"
"Steve Fox SharePoint IIS Media Video"