This may be a conflicting requirement because the protocol isn’t well defined and based on erroneous assumptions.
Ask ourselves about some BT.709 red stimulus and what should happen. We could have 15 units of light stimulus of pure red, and then 10372722 units.
The whole discussion of why the dechroma must occur has been sadly avoided, and as such, it is possible that the reasoning of an “either or” situation is erroneous.
Until someone can locate what to do in the above example, and then the “why to do”, it’s an ill defined problem with ill defined speculation.