Artists-at-the-Ready

I don't believe it is. The original creator cannot be reached and as far as creating a derivative work based on AATR it would probably depend on what you called it and if you referenced other artist's work or not. There's a lot of gray area. For example, if you participated in AATR then any story you wrote would technically be yours and your "version" of any characters used in it could potentially be considered usable since by participating any artist acknowledges and agrees that other artists are going to reference their characters.

Personally, I wouldn't write something I planned to sell based directly on AATR as a "brand" but if you were just writing to publish for free I don't think there would be a problem. Well, at least not legally speaking. Some people are sensitive about this kind of thing so you could run into problems if you referenced another artists characters and they had a problem with it (but again, if you both had been in AATR, especially at the same time, there should be some expectation that their characters could be referenced.)

Another consideration is that since we are not currently in an AATR season and/or if you have not participated in AATR before, writing a new story based on AATR or any associated characters could be considered unpermitted use. Although to be honest, the core concept is vague enough that you could easily write something "in the vein of" AATR without referencing it at all, which if you were doing it for profit is what I'd ultimately recommend.