How about you get 40 developers at the event to flash their own boards... if you are lucky you will then have forty boards updated at the same time.
That's how its handled at the IoT Roadshows. That way they have the knowledge and procedure to update the board when the next s/w image is released.
I have enough trouble doing one at a time... If you find a way to flash 40 please share because for some reason I have not given up on the Edison yet, and I may have the need to do the same one day.
Unfortunately you will have to flash them all one by one. I mean, I'm not saying that it is impossible to flash more than one board at the time but all the flashing methods available were designed to flash one board at the time.
You also would like to add some packages after flashing the board. You could avoid that step by creating your own image by following the steps in the BSP. That way you can flash the image with the latest MRAA, UPM, etc. already installed.