All Public Call prompts/requests are now public on AO3.
You can write treats (additional stories) for the participants whether or not you're signed up for the challenge. Just pick a prompt, write, and upload your story to the Public Call 2013 collection, gifting it to the recipient in question.
We're revealing fest entries one a day, which will take us through nearly three weeks of December. This is how long you have to write/upload your treats: Treats will be revealed on the 20th.