All Public Call 2015 prompts/requests are 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 2015 collection, gifting it to the recipient in question. (At the end of this post is a guide for posting treats to the collection.)
We're revealing fest entries one a day, which will take us through three weeks of December. Treats will be revealed on the 22nd - that's how long you have to write and upload them. :)