I didn't create a separate post asking for questions, but I added a banner to the end of some of my pages letting people know about the upcoming Q&A event, and asking them to post their questions in the comments. I also put the notification on my social media and asked some friends, so I got a wide variety of questions. I only added the banner to every other episode, and only posted it for about three weeks, so it didn't overwhelm any of my readers who didn't care about that sort of thing.
Then when I posted the results of the Q&A, I made sure it was a bonus update, not a replacement of a regular update. So I past pages on Mondays and Wednesdays, and I posted my finished Q&A episode on a Friday, so it wasn't a punishment in pushing back a proper page update, it was an extra fun bonus thing.