Well, I tried the same and I didn't saw files from brcm folder. But maybe I made some small mistake. If you are saying that it works then it is ok.
But you don't see files/folder from /storage/.config/firmware on /usr/lib/firmware/ right?
It's a bit of a symlink jungle, easy to get lost (maybe I should clean that up a bit some time):
/lib/firmware symlinks to /var/lib/firmware, a directory in a ramdisk
/var/lib/firmware gets populated with symlinks to the various kernel overlay directories, from /usr/lib/kernel-overlays and/or addons.
Currently only /usr/lib/kernel-overlays/base/lib/firmware contains firmware files and that directory has a symlink "updates" pointing to /storage/.config/firmware
Thinking about it we may run into problems when we add firmwares via overlays (which we don't do at the moment) as that may create symlinks in /storage/.config/firmware. Tricky. maybe handling .config/firmwares via a special case in the kernel overlay system may be better - I'll think about it some more.
so long,
Hias