Ok, 2 part question.
The UART bootloader defined the "special" and "extended special" commands. Those are nicely integrated into OpenBL. So if I wanted to write my own custom "OpCode", what range would I use ? Looks like some are used for STM32WB (FUS, documented), and some are used for SFI (not documented).
While "special" and "extended special" are documented for UART / I2C / SPI / CAN / FDCAN / I3C, there is no such documentation for USB. For STM32WB there is some simplified support for FUS commands, but nothing that would map the full feature set of said commands. I had expected something there assuming that SFI is done throu those commands, and since SFI is avaiable via USB, it feels like this is just a documentation issue.
