This morning, I had an idea for a new approach to this that I think could work well for us.
The basic idea is to introduce a limit to how many open questions can exist at any one time. An “open question” in this case would be defined as any reply that is after the latest post by the original. So if the limit were 1, then the original poster would have to make every other post.
This would allow for a “continuous AMA” with the orgiinal poster, but with an implicit rate-limit. When they stop answering, people cannot keep asking.
Implementation-wise, there could be a custom UI behind the gear icon in the composer for a new topic to “create AMA” and set the open question limit.
Or, alternatively, perhaps it could be done via a site setting that associates a tag or category to moderate in this fashion (in which case, the open question limit would be site-wide).
What do you think?