The bitcointalk channel will thrive or fail based on the number of people who view it because of their passion for the forum--that's my opinion. I highly doubt it's going to attract an audience outside of that (unless it becomes something other than a channel focused on the forum), so all of that standard nonsense mentioned above is unnecessary. A member like me will subscribe and will hit the like button, and you won't have to ask people to because there are many members like me who'd be looking for new videos constantly.
Focusing on Bitcointalk at the very start is a good idea, because you are pretty much guaranteed some sort of viewership which will aid the channel to grow into more. Although, I do see it becoming more than that, and perhaps its own thing. For example, if the channel interviewed various users from the forum, but also people that have played a significant role in Bitcoin, then they could potentially do a sort of question, and answer thread here on the forum as an extension, which might not be the primary focus of the Youtube video, but could potentially encourage any outside viewers to visit the forum, and then potentially be retained by their curiosity, and the quality of the forum.