On MacOS, run ioreg -l | grep RequestedFiles
Among the files listed, grab the trx, tle clmb and the txt. In BigSur, they live in /usr/share/firmware/wifi.
Rename them all brcmfmac4378-pcie, with the extensions as: trx -> bin, clmb -> clm_blob, txt unchanged.
In a temporary folder, create a directory structure lib/firmware/brcm/ and place the three file in brcm.
Create a cpio.gz archive from that folder, and use as initrd - or add to your current initrd.