I'm also looking at Adobe Document Center. Don't know why I didn't dig deeper into this one...
The AdobeŽ Document Center is a web-based application for controlling and tracking documents. Subscribers can control access and availability of documents through security policies managed by Adobe LiveCycleŽ Policy Server.
Policies solve the challenges of sharing files that are time-and version-sensitive and for content directed to a limited audience. As a subscriber, you decide who has access to your files and for how long, and whether recipients can copy, print, and add information. You can also track usage activity on files and revoke access to files when they are obsolete.
The most significant difference between the Document Center and password- or certificate-based security measures is that policies offer persistent protection. Any changes you make in the Document Center are updated in distributed files.
I do believe this is going to give my customer the ability to do "everything" they wanted to do with their document library. And I do mean everything. ;)