So the MS SQL Server database is overly complex? Hard to believe that the image data is not relatively easy to find in there.
The SQL database appears to be fairly simple. However, it's not in any easy-for-a-human-to-decipher structure (at least this human).
For what it's worth, we used to have a system that used IBM's FileNet for document storage...and I easily reverse engineered the Oracle back-end of that and was able to pull docs from that with no issues.
This is nothing like FileNet, unfortunately.
Of course, its so you pay them to do whatever customization you are after.
Sadly i have no experience with Apache Jackrabbit. Hope you figure this out!