Hi Stokerg,
I actually have a license of Affinity Photo and did went through the documentation and online videos.
From what I see, the configuration is evaluated once and the interface only gives access to the Display transforms (as a combo-box selector in the 32-bit preview panel) and colorspaces (as a correction layer that applies a colorspace conversion).
If Affinity Photo had a scripting API and was exposing the current OCIO configuration, I would be able to update the variables through a script but I don't believe that's an option for now.
In any case, it would be better to offer a way for the user to re-define values of theses variables on the fly , and in that regard, Nuke's solution is a good inspiration: