we tested etherpad, which hackpad is built on (and google docs is) and it's just not super stable if you have 50+ folks on it.
I've spoken with the founders and from what I understand they basically gutted and rewrote Etherpad. It's been used at both Pycon and SXSW which I'm assuming was pretty heavy load.
It's great for 10 folks, but I need a 10,000 person solution. sigh, i guess i need to build it.
hackpad op here. give me a ping, i think we can support you.