The Be operating system (BeOS) uses BFS as its native file
system. BFS is a modern 64-bit journaled file system. BFS
also supports extended file attributes (name/value pairs)
and can index the extended attributes, which allows it to
offer a query interface for locating files in addition to
the normal name-based hierarchical interface. The
attribute, indexing, and query features of BFS set it apart
from other file systems and make it an interesting example
to discuss.
So yes it has been done. Personally I like the AS400 idea most - an object store where you don't even have to really "think" about the difference between memory and storage - you just have objects that you can cause to "persist" when you "commit" them and anything they are linked to by pointers will also be persisted.
Also, prior to BFS, in the earlier beta builds of BeOS, the file system was much more database like, but it ended up having performance issues (IIRC), so in a later version, they replaced it with BFS.
Apparently the earlier builds used an actual database alongside the filesystem but it was hard to keep them consistent with each other so they altered it.