Sure sounds awesome but it would be better to have a champion scheduled per week at a fixed time to run the chat... Liz over to you. (I like your idea and augment it with my own - to paraphrase Adam Savage of Mythbusters http://www.youtube.com/watch?v=W8qcccZy03s)
A nice idea in principle but most companies do already have chat/instant messaging capabilities outside of Marketo so I am not sure how much extra value this would really add.