Good idea, and to keep it more real I think you cannot allow use of the "auto" function no matter what it is exactly called. Instead make it mandatory that you use the manual mode and use the exposure guide provided in your camera viewer only. And pick one ISO setting to use, like you would have picked a film speed.

And to make it a bit more realistic, make the rules that you have to wait three days to see what you captured, sort of like it was in the day when you had to wait to see your prints. This rule would almost guarantee that the moment will have passed so you got what you got and couldn't retake the shot.

I'll leave it to the creator of the idea if you want to implement any of these rules, LOL!

Wayne