Reddit is also good to sprinkle on to get people to the hangouts
It is so ironic. I have a reddit page and used to direct people there to ask questions...etc. but noone was interested in using it. Maybe there is a way but I am not sure at present how...
Have you tried using your signature here on the forum/BTT, updating it to reflect the new hangout topic/guest/date.
You post in a lot of threads, so you would get exposure to all of those threads, instead of just one thread.
I'm not sure of the signature rules here, but I don't think they allow placing a signature within each post (above specified signature), but if they did (or made an exception), you could then post links and make it a little more noticeable. Since you're promoting BitShares and talking with Dan, I don't see a problem with allowing you to have a sig with links at the bottom of each post to promote specific hangouts dealing with BitShares related topics.