I don't know if the ~$20 HDMI USB capture cards work on a raspberry pi, but connecting one to a pi or laptop and capturing the signal to a v4l stream, overlayin

I don't know if the ~$20 HDMI USB capture cards work on a raspberry pi, but connecting one to a pi or laptop and capturing the signal to a v4l stream, overlaying in software and then outputting on built-in HDMI out might work?