I just pushed fdb.py 1.21 to GitHub (http://github.com/njr0/fdb.py).
The command line utilities and the API now work consistently with subnamespaces using the funcitonality introduced in 1.19.
So
fdb tag -a DADGAD /njr/rather/deep/space/rating=9
will now work, creating namespaces for as many of rather, deep, space and rating as required.