I understand the cost and everything, but couldn't it be something every tuesday from 8 to 10 or whatever, with some kind of schedule, like "this tuesday, ask Jon how he came up with the ideas for this site..." I hope everyone here will try to say what he/she thinks...
Can't you use some chat rooms in major sites? For example in france, you can have a private room in wanadoo (the biggest internet provider), and you can have a link to the room on your site. All it takes is having a bot guarding your site, and you can even let it be invisible to other users, so that anyone can connect from the site, but only from the site... Maybe I'm saying stupid stuff (I don't know a lot about all those stuff) but I think it would be interesting.