Config Flow with YAML for "very advanced" options

Is there something in helpers or core that would let a custom integration use YAML config while still using Config Flow? Or do I need to implement my own YAML config stuff outside of HA core? When I read ADR-0010 I interpret to mean that Config Flow and YAML can’t coexist, so once you implement Config Flow for an integration you need to migrate away from YAML permanently.

My goal here is to have some very advanced options I don’t want to appear in config flow UI screens. For example, if a user wants to use absurdly long timeout values that I would never allow into a release, but if they change the constants file it will get overwritten. If those type of users want to dig in and change settings like that I want to give them an option to persist across updates, but I don’t want regular users seeing it to be easily (or accidentally) changed through the UI.

The trend integration can be set up from both YAML and the UI, so it should be possible for custom integrations too.

Thanks, I’ll look at the source for that one.

Setup config entries or YAML is mainly “independant” of each other so therefore you can do both if you prefer.
For core integrations there are rules in play which you have already mentioned.