The parameters can be set by the bootloader and a digitally signed. There is no need to make 3 different chips for 3 different units. Just put the parameters in a payload with the target serial number then digitally sign it.
Then in secure code (either in ROM or loaded from flash by a ROM and checked before running) you load those parameters into the radio before proceeding.
This would add no cost (or trivial at best). All you need is an unchangeable unique ID. Everything else can be in the existing flash storage. It would add some complexity.
Why would a manufacturer do this? Because the FCC would mandate it.
You do not need a separate firmware for the radio, you design the radio so that these values become read-only after set. Then the entire driver can be modifiable (open source) it just can't modify that data.
This can be done relatively simply and for no additional cost. So no, the FCC wouldn't be banning open source, simply changing how the systems which use open source must work. And in a way that is really easy to roll out.