Back when I was first getting into all this, my only HDMI source was a DVD recorder/player. When I connected it to a BMD HDMI to SDI converter, I got nothing. I’ve seen blue screens and black screens, but never static. (At least not yet.) I’m using one computer to do chroma lower 3rds and play videos via a VLC playlist. That means we can’t do them both at the same time, and switching from live to pre-recorded and back is more hectic than I’d like.
Dedicated devices are good, and a £24.99 DVD player beats a $700 HyperDeck if it works. You might consider a USB/SD player like the one @aaronpk showed in his video. Micca? I’ve got one with a different name, but the interface looks identical. I’d prepend a short title/countdown to the beginning of the video so that the video player interface has time to disappear before getting streamed.