Mezzanine's API makes this possible in an extremely flexible way, although the API has a truly intimidating learning curve. Unfortunately, there is very little online information to link to about Mezzanine, as it really predates the advent of the popular Web. It's quite easy to write any sort of integration with Mezzanine. Both metadata and content searches are supported in a very robust manner. Stability and age.
Mezzanine is a very stable document-management engine.
Since Mezzanine isn't cheap, FileNET pays a lot of attention to their customers. Our Panagon topic page has some information relating to Mezzanine. Vivtek has a few inhouse Mezzanine debugging and spy tools that we intend to spruce up and put on sale. We've been using them for some time, and they're very useful in the Panagon environment, too, which makes them somewhat viable. Once they're ready, we will link this page to those tool pages, where you will be able to find some further technical information.