The problem with attempting a temporary repair is the area has to be clean and oil/ATF free. That means sanding it and removing the oil, once you sand it to prep it, it will leak more. You can try cleaning the surrounding area and not the rust itself. Get a piece of lead flashing, and JB weld the flashing to the cleaned area of the transmission pan, making a tight seal, and keep your fingers crossed and hope it doesn't leak, or leak worse.
Several years ago I repaired an oil pan on my Aerostar [thanks to member eljefino] using this method. But I was able to allow the oil to drain overnight before cleaning the area and applying the JB Weld. You don't have that luxury unless you drop the transmission pan, in which case you could probably do a much better repair until you get the replacement pan.