The Mt. Okemo Hillclimb is put on by the Killington Sports Car Club, not SCCA. Therefore the rules are different. Check out this link for more information:
http://www.hillclimb.org/events/okemo/okemo_info.pdf
My parents live at the top of the Weatherly Hillclimb. I've been involved with it, and several cars that ran the local SCCA hillclimbs for years. I am pretty familiar with them and their rules. I do know one person who goes up to Okemo and runs that hill. What was said earlier is right, you do not need a cage or bar in the car, but they have a time set that if you go faster than it you get told about it, and then if you do it again you are done. What my friend has told me is that there are a handful of cars that just go flying up the hill, but then let off towards the top to go slower.
One of the other interesting things of Okemo, vs SCCA hillclimbs, is that you can take passengers. Not exactly sure on the ruling for it, but I do know you can take others along to help, or maybe just see it.
Check out this video, very fast car that when you saw it you would never expect it;
http://www.youtube.com/watch?v=Pq3hqtfIUEA