What about each participant shooting one roll in one location in one hour, then presenting a contact sheet and at least five selected images?

Then we could use just about any camera, there would be some leeway for "mistakes", and it would be each participant's own choise of film - but also each participant's responsibility to develop and print the film...

We would have to decide in advance whether to use a 135 or 120 camera, since not everyone can print both sizes.

I might offer up a Voigtländer Perkeo for something like this.