A type of mixin allows to you to access variables that are not visible (private) or final:

@Mixin(StructuresConfig.class)
public interface StructuresConfigAccessor {
    @Accessor("DEFAULT_STRUCTURES")
    public static void setDefaultStructures(ImmutableMap<StructureFeature<?>, StructureConfig> defaultStructures) {
        throw new IllegalAccessError();
    }
}

The error will never be thrown, you can use the function.