That’s a different kind of play than what I mean by “forward progress”, right? The data required (when did the knee touch the ground) is contained in the video. So if you have a tracking system using video for ball location you can just tag the frame where the knee is down and then use the ball location data to see if the first down was reached.
For forward progress (as I mean it) you would need additional data - when the referee determined forward progress was halted. What I’m unclear on is what the determining factor for that is. Is it when the whistle is blown? Or do the rules assume there is some delay between when the ref makes the call and when they blow the whistle? If it’s the latter then there is really no way to review it using tracking tech. I suppose if it’s the former (whistle) then you could try to sync the whistle to the video and use the ball tracking data at the time of the whistle.
What you can’t do is just track the ball and if it ever crossed the first-down line to gain call it a first down. Because the play ended at some point, which could be prior to the furthest the ball moved forward.
I suspect that the NFL is reluctant to go to fully automated ball-tracking for these reasons. It injects a veneer of accuracy that really isn’t appropriate for what is actually happening on the field. Similar to the automated offsides in soccer or (possibly) the automated strike zone coming to baseball. Fans think they want that kind of accuracy, but there is a decent argument to be made that they really don’t. They just want to obvious mistakes fixed - not the little ones.