For now this is my conclusion on how to access the required value from withing the MinecraftServer.class

    @Override
    @Nullable
    public ReloadableServerResources codec$getResources() {
        try {
            Field resources = MinecraftServer.class.getDeclaredField("resources");
            resources.setAccessible(true);
            Method managers = resources.getType().getDeclaredMethod("managers");
            managers.setAccessible(true);
            Object reloadableResources = resources.get(this);
            return (ReloadableServerResources) managers.invoke(reloadableResources);
        } catch (Exception e) {
            return null;
        }
    }