Okay, I’ll have a go at organising this
Here’re my current thoughts on how this would work:
at midday friday (GMT), a new thread (eg. “Weekend Challenge Entries for 10 May 2002”) is created in the Contests forum, giving the new topic/theme for the weekend, and maybe some how-to-play instructions. Contestants post a message with a link to their entry in this thread. If someone posts more than one entry, only the newest is elligible. That is, one entry per person, but the entry can be updated. To keep this tidy, posts without entries (eg. comments) would be discouraged in this thread.
at midday (GMT) the following monday, another new Contests thread (eg. “Weekend Challenge Voting for 10 May 2002”) is created, containing a poll for everyone to vote for their favourite entry. Comments are welcome here.
at midday (GMT) the next thursday, the entry with the most votes is declared the winner. The winner is contacted via email/private message for congratulations, and to request a new topic/theme. Since the winner provides the next topic, they’re excluded the next challenge.
This seems to me to be the simplest way of doing this, requiring only two thread creation posts (moderator assisted, since they’re in the Contest forum), and one winner announcement post per week.
Additionally, the organiser can participate too, as they neither select topics nor judge winners.
How does this sound?