For some reason, I wasn't able to access the collar drawing/writing features (the top left of my screenshot SHOULD have links to those features). I was able to access them fine the last time I did a Hangout, so not sure what went wrong. BUT at least everyone else had fun experimenting with the collab tools.
Apart from that glitch, however, the main reason I can't fully recommend Google+ Hangouts for a public event yet is because there is currently no way for one person to moderate. If someone behaves inappropriately (I had someone expose himself in my previous Public Hangout!), there's still no way to kick that person out of the chat. You can block him, but that only means YOU don't see/hear him anymore -- everyone else in the chat will still see him unless they block him as well.
Still, it has great potential for creative collab with trusted invitees.