DpkgRootfs and RpmRootfs do not read the ROOTFS_POSTINSTALL_COMMAND variable. Noticed this while looking at poky/meta/lib/oe/rootfs.py I am using ROOTFS_POSTINSTALL_COMMAND to run my post processing script (to update file timestamps to a known consistent time) before self._run_intercepts() is called. I am using OpkgRootfs so its not an issue for me, but ideally this should be fixed for dpkg and rpm too.
As noted, ROOTFS_POSTINSTALL_COMMAND was run only after the opkg rootfs install post processing phase. Submitted a patch to the OE mailing list that makes it generic so it is run after any package manager specific rootfs post processing, but before _run_intercepts().
Fixed in oe-core 8f2571881cd01ace88d282a63ad802cab4ab6940.