I've been working on metadata backend that would use tdb (meta_tdb). It's 
advantages over meta_fs include:

- proper handling of "metadata" named files and dirs,
- compactness (no directory/files structure overhead and storing data in 
binary),

Performace is yet to be tested extensively and compared to meta_fs.

There are also additions to humfs features:

- humfs stores file/dir access modes now (fixes some chmod problems),
- humfs automatically initializes mount dir if only valid "superblock" file is 
present in it (no other files, new humfsify is not yet ready ;),

Example "superblock" is attached. To try meta_fs backend with it just change 
"tdb" to "shadow_fs". To mount humfs:

mount none <uml mount dir> -t humfs -o <host dir>

The patch is against UML release ver. 2.4.26-1. This release is of beta 
quality (should mostly work ;), but please note:

- database format is subject to change (right now db keys may collide, though 
it is very unlikely, I coded new fixed structure but it's not fully debugged 
yet),
- meta_tdb does not support unmouting and remounting (since metadata unmount() 
method is not called by humfs, this is no issue for meta_fs becose of the way 
it operates).

The patch is also available here:

http://kem.p.lodz.pl/~peter/humfs-2.4.26-1-meta_tdb-1.patch.bz2