Great question Georg. Sorry to report that I don't have any experience with such a thing.
It's an interesting idea, but in my experience from running webinars (like those that we have run here on UX Mastery) I could imagine that there is a risk of participants feeling apprehensive about contributing to the conversation. And would you require all participants to turn on their video/microphone at the same time? I don't know about where you are, but it's rare to find an internet connection in Australia that is fast enough to handle that kind of video data being streamed back and forth. @HAWK, @Lukcha and myself catch up on Skype once a week, and we often experience problems with the connection with only three of us.
So while I'm not ruling it out as something I'd try, I think that there are a number of reasons why it could be unsuccessful (how to put participants at ease, how to encourage participation, technical infrastructure, and the opportunities for collaborating with sketching/post-it notes, like you mentioned). If the participants that you want to interview are not in the same location, I'd personally schedule one-on-one video chat interviews with them instead. That way you can reassure them, and they won't feel apprehensive about having their face visible by every other participant.
I also thought it was interesting that you said an online focus group would be quicker than an in-person one. What makes you think that's the case? I agree that it means you can cast the net wider, to include people in other cities/countries. But in terms of the time it would take to organise and run, I would have thought it should be about the same (assuming attendees live in the city and all come to your office).
As an aside, I'm not a big fan of focus groups in general. I worry about "group think" and strong personalities dominating the conversation. I always find individual interviews to uncover more insights, as it's a better environment to get to know someone and what makes them tick.
However, if you do decide to try it and have some success, I'd love to hear about it. In fact, I'd consider publishing an article and creating a whiteboard animation for the techniques bank about it.