I'm working on reproducible builds in Libreboot.
I have source tarballs generating reproducible now; ROM images are not yet reproducible.
Patch: https://browse.libreboot.org/lbmk.git/commit/?id=e0feda639b0112bd6a40e4caee168ea3b82b10e4
As of this revision, I could do this:
./update project release -d /some/path/of/my/choosing -m src
and if i did that several times, it always got the same hash, on each same revision.
Some polishing (and more testing) still needed, but it's a start!