[giantrobot]

For Part 15 devices the licensed portion of the device is the software and hardware that does the RF emission. Firmware defining operation is seen by the FCC (and other regulatory bodies in other countries) as the same thing as a circuit design of a fixed radio transmitter. To the point of firmware updates can require recertification just like a change in the BOM or circuit design would.

A RISC-V radio baseband will end up no more open than a Cortex-M one. The CPU core portion might be documented/open but not the full firmware. It'll still be effectively a black box no matter how open the specs on the processor.