The idea would be to announce in advance when this will happen and open the forum up to the guest lecturer for a period of 48 hours in the middle of the week (that's 2 days for us mathematically challenged). That should give all the forum readers around the world enough time to weigh in and ask questions, and the guest will know they only have to follow this thread for 2 days, not 2 years (or longer). After 48 hours we'll thank the guest and I'll lock the thread to stop further discussion.
Regarding format -- occasionally, Slashdot
(one of the original crowd-sourced news aggregators) will feature an interview where the crowd can ask questions. The way they do it is they create a new post inviting questions for the interviewee. Sometimes it's on a specific subject, sometimes the floor is open to anything. After a couple of days, the most upvoted questions are passed on to the interviewee; a week or so later the interviewee's responses are posted as a new article.
Maybe you could adapt that approach. Of course, we don't have an "up/down-vote" process here, but that would be a way of minimizing the amount of time the guest would have to spend here. Then if they wanted to, they could respond to followup questions.