On a massive scale, this takes too many people to pull off. More likely that people on the inside would be the ones who could accomplish this in large numbers.
I do think there would need to be someone sweating it out in a back room, but it seems to me some study would have to be made to figure out who regularly doesn’t show up.
You could do it after the pools close, or before the polls close with willing accomplices.
It would be easier if it was after the polls close, but if there were Republicans around, the accomplice before the polls close would be the best bet IMO.