Thanks for the suggestion; however, this is a video production device which aims for different functionality than the broad one I had in mind.

The simplest useful app would be: watching a movie together with friends, have them type their comments on their phones and display them on my TV, in a lower third. The chat part is trivial but how to get that from a server to the video stream? Maybe there’s a simple way to do it using a device like the Mini Pro but when I search I do t see it.

While it's not the same as what you're asking for, you could do this pretty easily with a Raspberry Pi.

That feels like such an obvious thing to say that I hesitated even saying it, but then I started thinking about how funny it is. A solution to HDMI being so locked down is to buy a $20-ish computer, that does all the things a computer can do, so that we can feed the TV the video stream we want. Over HDMI.

Given that it's so easy to circumvent now, I wonder why anyone even bothers with HDMI any more, aside from inertia.

Totally, or one of those hdmi usb capture devices that are similarly cheap

The Atem can definitely do this. There are youtube tutorials on it.

You use one of the many apps that can output your text, set it to output the background as green or whatever.

Then you can use the Atem to do one video stream inside the other, picture in picture, you can customize the size of the picture in the picture.

You can also have ot chroma key out the green (or whatever) from the text app, so it will only display the text, and the background will be the main video stream.

Bam.

> watching a movie together with friends, have them type their comments on their phones and display them on my TV, in a lower third

This is what twitch streamers do all day using OBS and twitch chat. Hang out with their followers and watch/react to YouTube videos and read chat comments.

There certainly is a conspiracy of industry players to keep people down. It is well documented and sadly part of our devices.