The fastest method to attach hex nuts to sheet metal would be to modify a stud welder. A commercial stud welder is grounded to the metal plate and the threaded stud is held in the opposite electrode. Applying pressure to the stud against the plate closes a switch, dumping a rather large current through the stud, effectively spot welding it to the plate. A quick search returned a custom-made nut spot welder video of about a minute length. The same search returned a number of "spot welding nuts" videos and related links.
Note in the linked video, the square nuts have convex bottoms, causing the corners to contact the surface first, concentrating the current in those locations, ensuring that a weld is formed.
A flat hex nut would require substantially more energy/current than one with, say, a triple point profile.
Another answer is brazing, as one can place the nuts are required, on a surface pre-coated with flux and apply sufficient heat to secure the nuts. The brazing would not require the entire sheet to be heated at once, but the heat can be applied in a moving wave. Big torches, plenty of gas, I think it could be done.