I'm 100% certain that most professionals are already using these for performance displays. I don't know a single person that doesn't use these cards for exactly this reason. We even use one for presentations/events at movie theatres just to make sure that nothing is going to put an errant pop up on the display.
You can render on your GPU to a texture, then stream that texture to these devices. No different to rendering to your screen and then streaming and saving to disk, as you would do if you were screen capturing.
No. If they did, that would defeat the purpose. Usually, they have their own setup software or routing software. Something like Blackmagic, for example, is supported directly from certain apps. Others let you route windows to the output. You still need a graphics card for these to work. They only handle the video input and output.
Though a quick search through online forums suggest that module isn't compiled into Ubuntu's version of VLC by default, so you may need to compile your own version:
Not sure about VLC, but ffmpeg has great support for Blackmagic, you just have to download the Blackmagic SDK, compile ffmpeg with Blackmagic support (and the SDK in path) and then you'll have a separate input/output device available in ffmpeg. The other great thing about this approach is that this way audio also takes a dedicated, integrated path, bypassing OS layers and maintaining sync with much less effort.
I'm not terribly familiar with Linux use of hardware I/O devices so my guess would be yes unless Blackmagic (or someone else) makes some kind of Linux kernel extension that would allow you to manually specify that or VLC has built-in support for external I/O devices.
Edit: Just checked because I was curious. The default installs of VLC do not include Blackmagic support but you can compile versions from the source for Linux that include it (if you also download the support files from Blacklink first).
