@jgallo We must be aware of a few things here.
The github site always contains the latest release. So (currently) the files contained are for FOG 1.5.0. If you are working on a down level version of FOG then you need to take the route of extracting the inits. You don’t need to repack the inits, because you only need the file you want to tweak. Once you have the file you want to patch, then you may unmount the loop directory and remove the unpacked init.
The structure of files may change from release to release. In the case of FOS, it was split out into its own branch. The correct path (as of 1.5.0) to the file in question is:
https://github.com/FOGProject/fos/blob/master/Buildroot/board/FOG/FOS/rootfs_overlay/bin/fog.man.reg (full disclosure, it took me a bit to find the right path too).
The only caution is as newer versions of FOG are released, your patch file may become stale, where you might need to update a newer versions of fog.man.reg as newer versions of FOG are released. The risk is that you might be applying an older version of fog.man.reg over an updated version fog.man.reg breaking FOG without a clue why.
The rest of the instructions by @JGallo are still accurate (at least for 1.5.0). You can still use the post init scripts to “patch” the current FOS image as in the original post.
