I don't think it'd be possible to run Yuletide in particular under the terms you suggest, just because it's a gift exchange based around the holiday season. It's not an easy-going type of challenge, you know? And some of us need that kind of hard line to give the kick in the pants we need to work through our writer's block.

However, I think even Yuletide does try to be open to other levels of participation. You can fill NYR requests from January through the rest of the year, and that doesn't require you to commit to anything, you just browse the prompts and upload sometime before the next round starts. If you don't finish the story for whatever reason, no worries.

I've seen lots of communities do prompt fests and things like that that might meet most if not all of your conditions.

These are good things to keep in mind to make challenges more accessible for a lot of participants, so I'm glad you posted about this, thank you.