My tuppence worth:
I would think that you can't count on people all joining in before hand so I would just use moodle as a set of resources before the trip, maybe 2 weeks? I would also leave the students a rest week after the trip before getting them to take part after the event, then run for not more than a couple of weeks. This gives those with previous committments chance to skip a week if necessary. I really like this design, let us know how it works!