There is a feature request at sysvinit bugzilla for root unmounting support.
Created attachment 386004 [details, diff]
Created attachment 386006 [details]
I've also mailed attached patches and scripts to sysvinit-devel mail list, but it looks dead:
Patch is slightly different from the one attached to the mail, it has different file names. Patch attached to this bug is modified to be accepted when it's put to "/etc/portage/patches/sys-apps/sysvinit" directory.
Attached scripts just unmount root and proceed to powering off or rebooting system. For debugging purposes interactive shell is started right before reboot/poweroff. Just exit from it to continue to rebooting/powering off. Scripts are just an example, proof-of-concept, but they may be used for implementing further features, like shutting down encrypted tomes or lvm or anything else. Scripts require statically-linked busybox in order to work and they assume that they are put into directory "/usr/local/sbin".
WARNING: I've tested scripts at VM and they worked fine for me, but for your data safety read them first and try them on spare VM first.
Seems like a nice feature to me. systemd is already capable of doing this, but it might be useful for those that prefer to stick with the old init/openrc.
I would suggest adding some documentation to the inittab.5 manpage.
Also, a better summary for this bug would be "Allow replacement of PID 1 via exec action in inittab".