Lennart Talks Up The Power Of systemd-sysext For Testing /usr Changes
Systemd-sysext allows merging one or multiple "system extension" images into /usr via OverlayFS. This is done instantly and atomically at run-time for modifying the exposed /usr contents of the system. These system extension images can either be (optionally signed) images or plain directories located elsewhere on the system. These /usr changes aren't persistent and can be unmerged during run-time or otherwise lost at reboot.
Lennart is blogging about systemd-sysext for shining more light on this effort. Among the many possible use-cases for systemd-sysext are for loading debugging tools on an otherwise locked down operating system without developers needing to build in said debug components. Lennart also personally uses the systemd-sysext method himself for testing systemd changes at run-time right away. There's also other possible applications for making use of systemd-sysext for testing /usr changes by developers.
Those wanting to learn more about systemd-sysext can stop by Lennart's blog and the man page.