Nobody has to chair it, or the op of the thread could. A thread is created and anyone can ask a question in there. People can read the thread to see if there's any dupes. Whoever is nominated can either choose to answer as many as they want or all if necessary and they can answer them when they're ready. As with AMAs not all questions get answered but just ones they feel penitent and they certainly wouldn't be an obligation to do so. I think this would be better than an reddit AMA actually as questions likely won't get lost amongst the melee and instead of answering them on the fly and in a rush the person in question can take some time to think about their answers or what questions they even want to answer at all.