You might be interested in a project I was working on.  Arch linux
already has something similar, I added it to Gentoo's sunrise overlay,
and the other major distros are too intimidating to submit it to
without better integration, which I've little inclination to work on
without a known path for inclusion.

It builds an initramfs and/or initrd using local binaries and is very
easy to customize since it builds the image within a subfolder.  You
can even chroot in to it and run informational only checks to make
sure you've included the things you'll want.  Plus you can work in a
subshell and then 'exit' to make a change like this or run an fsck
before early-userspace ends.

https://sourceforge.net/projects/aeuio

If it doesn't work please let me know so I can resolve the error.