Interesting, not sure about an online hackathon though. I participated in several hackathons during University and most of the fun was the atmosphere working with others and spending a short period of time like 24/48 hours with almost no sleep on something that will probably break hilariously right as you are going to present it (like the time we built a thing that had a feature that generated a word cloud from live tweets in a geographical area. This feature having been cobbled together at 5am on the morning of the judging day. Of course during the live demo we ended up standing in front of a word cloud full of expletives...). Oh the stories I could tell from hackathons I've been at... This element would be completely lost if it was purely online.