Ampache support would be a killer feature. owncloud http://owncloud.org/ now uses it http://owncloudtest.blogspot.com.br/2011/08/new-in-owncloud-overview.html.
It would be great if this great player had owncloud support so I could use it after I set up my owncloud.

Sorry for not replying earlier.
I'm not sure it's a good idea, it severely limits what can be done with the files.
Though support for that kind of external library is more or less planned, I wonder why exactly would you use that rather than mounting your owncloud files via webDav (http://owncloud.org/support/webdav/) and adding them as normal files ? (I haven't tested but it should work)