You should be able to do most of that via plugins. See
MogileFS::Plugin::MetaData perhaps as a start. Maybe you could do
something more efficient?
Either way, it would be welcome. It might be hard to pull in as a core
feature since we want to avoid specializing, but plugins can work well. If
there's a hook you need or core changes to make it work better, we can
certainly accomodate that.
-Dormando