Maybe I can help you, but I don’t know exactly how the work is going.
There are several ways to use custom assets at runtime.
It depends on “Mount Order” of pak. If a resource with the same path resides in two packs, the resources in the higher order are loaded.
The first way, you can cook specific assets, make “Suitable” response files, make a pak that will overwrite origin assets, and mount it with higher mount order.
If you can handle C++, you can use custom order that you want by modify engine’s source .
But if you can’t handle C++ or don’t want to modify engine’s code, you can use rule of pak file name for unreal patch system.
The second way, you can use unreal patch system itself.
You can make “diff-pak” with using “project launcher” in unreal editor, and place it in pak file path.
This “diff-pak” will be named “*_P.pak”, and It will be mounted with higher order than origin pak.
If you want to use the first way, ask me questions by reply. But I recommend the second way, which can be known by through the Unreal document.