Introducing /run (a Filesystem Hierarchy Standard perspective).

Sharing /usr does not make sense when /usr is just one TLD out of several under the control of the package manager. Package installation and upgrading are going to be "interesting". It's also sharing the wrong thing; modern distributions allow / to be shared as easily as /usr, so you can just share the entire lot. If you so desire, you can use a unionfs/aufs overlay to make the read-only image writable.

Regards,
Roger