I'm very undecided about that package. Maybe it's a better way to discuss it again on pear-dev and then repropose it.
IMHO the idea of a kind of "registry" is handled by the Config package. Can you outline a bit more, what the difference is between your package and Config?
Another thing is, that the name "registry" is widly used on some specific OS for it's glo bal configuration database. Is the purpose of your package to deal with this?
Another thing is integration of multiple backends. Which are provided?