How to deal with custom SoC/SoM implementations

I'm creating a PCB based on the RPI Compute Module 4 and have ICs attached to it that I would like to expose in a CPY fashion as a pre-configured peripheral as part of the board definition.
So this would be a sort of flavour of an existing supported board ("bare" CM4).

This is something I have not seen addressed so far but would require a solution for in order to offer a good Blinka/CPY experience.
For me its specific to the CM4 but in general this holds true I guess for any other SoM of any of the supported SoCs (BeagleBone f.e.). I'm not sure if a generic solution can exist though.

Personally I would prefer it if an EEPROM solution like used for Pi HATs could be avoided as I feel these add unnecessary bulk. My board does not have an EEPROM connected.
My favourite solution would be to have a text file in` /boot` that contains an ID string that overrides whatever https://github.com/adafruit/Adafruit_Python_PlatformDetect/blob/master/adafruit_platformdetect/board.py determines.
This would be quite easy to do by a user manually or by an install script.

This is something that can also be done by a user on their computer or a host side install script after flashing a generic image in case of a Pi image (and others, I think BB also worked that way) as the boot partition is readable on any OS. So that would make it possible to modify an existing SDCard/mounted eMMC to work with a specific board instance without creating a custom image for people to flash which I think would be very bad thing to have in the ecosystem if board creators feel they need to maintain their own Raspbian flavor just to have a user friendly experience (which would eventually become outdated when they stop maintaining).