Thanks for your input - highly appreciated.
It appears you have a point regarding the variable assignment - 'bitbake-getvar OPKGLIBDIR' indeed indicates the intended assignment does not occur:
$ bitbake-getvar --value OPKGLIBDIR
NOTE: Starting bitbake server...
/var/lib
Doing the strong assignment (OPKGLIBDIR = "/data-os/opkg") does not make a difference, which puzzles me as all assigments (I am able to grep) are weak:
$ rg ^OPKGLIBDIR
meta/classes-recipe/rootfs_ipk.bbclass
30:OPKGLIBDIR ??= "${localstatedir}/lib"
meta/recipes-devtools/opkg/opkg_0.6.1.bb
32:OPKGLIBDIR ??= "${target_localstatedir}/lib"
meta/classes-global/package_ipk.bbclass
24:OPKGLIBDIR ??= "${localstatedir}/lib"
However, based on your reply, I am starting to wonder whether what I am trying to achieve is at all possible. The mount '/data-os' is a dedicated partition mounted R/W and used by overlayfs-etc, my assumption was to configure opkg to use this for the meta-data. But if the meta-data is rendered build-time during rootfs creation, I do not see how this is achievable?
Thanks,
Martin