Hi Rob!
/lib/firmware is part of the host OS filesystem, and changing it would either block Resin OS updates, or overwrite them on every update – As this could leave devices in a non functioning state, Resin OS doesn’t allow modifications to it by default.
That said, it is certainly possible to load custom kernel modules and/or firmware from a container. If you put the firmware in the containers /lib/firmware (instead of mounting the host OS’s), and load the required kernel module from there, that generally works well.
Alternatively, there might be a way to specify an alternative location for the firmware in the drivers?