I recall being asked to do an AMA on two YC-funded AMA platforms. (My guess: somebody told the founders "Patrick is in that happy intersection of 'people who have something interest to say' and 'people for whom doing lots of writing for free sounds really attractive.' Why don't you ask him?")
AFAIK, both companies no longer do that as their line of business.
Seems like if you are doing an AMA, reddit is the place to do it. I can't imagine any blog comment system working well for the task, and I can't imagine a way to build a profitable company that makes an AMA solution.
There isn't an effective embedded real-time "AMA" type platform that exists to my knowledge. Wouldn't be a terrible idea for a startup (but it could be argued that it's redundant with Facebook/Twitter)