Hey again, Sonic.
Thanks again for posting the video. I tried this with a USB stick containing many HDR demo videos. Funny thing is, the only video that gave me access to the AMP menu was Samsung's "Wonderland." Every time I played a different one, however, the menu would be grayed out; and it didn't matter if it was another Samsung demo (like "Chasing the Light") or a Sony or LG one.
Also, while this trick worked for the Netflix app, it didn't work for the Roku Ultra, which again reverted to Standard and grayed out the menu. This appears to confirm that as of now, any external device that outputs 4K60 only (i.e., no 24Hz option that also applies to streams) will lock the AMP menu and force it to Standard. Unless I'm doing something wrong, I don't think this trick can enable menu access for any model of Roku, Shield, Xbox One, etc.