Would you be into letting the person who puts the last grid on a level have the option of using an automatic phrase or generating their own? Maybe you could run the anagramizer and have that phrase show up in a modifiable box that the last person could edit before submitting the last grid and phrase.

But, of course, you're the one doing the work and have done a great job with everything else, so feel free to ignore this suggestion! It seems that there could be a problem with this scheme if someone clicked to upload the second to last image and then someone uploaded the last image before the second to last was uploaded. (Or something like that.)