As I understand it, visitors can bring their scopes and observe all night. I know the visitor center has facilities that can be accessed after closing. If I were to have a 4 wheel drive, how far up the mountain could I go? Has anyone taken a bottle of oxygen and a mask/nose clip so they can be comfortable all night at higher elevations? How does the seeing at say 9 or 10,000 feet compare to the summit? Would I be allowed to set up a small mountain tent?
I was just on Mauna Kea with a group from Astronomy Magazine as part of a transit of Venus trip. You will definitely need a 4WD to get to the top (they require it). However, visitors are not allowed at the top after dark. You can observe at the 9000 foot level at the Onizuka Center, but there is no camping and really nowhere to camp. When we were at the 14,000 foot level there was a 55 mph hour wind, volcanic dust blowing EVERYWHERE and a wind chill in the teens. We could barely stand up without being blown down. Couple that with 40 percent lower oxygen levels and it makes for a hostile environment. After 2 hours of being up there with David Eicher and Alex Filippenko we were ready to get the heck out of there!!! And this was early JUNE! Alex told us that even the professional research astronomers have to justify why they need to be on the summit and not in the relay station. There is only one public area that I am aware of and that is a tiny viewing area to catch a glimpse of Keck 1. No other buildings are open to visitors.
They have public viewing at the 9000 foot level every night and I believe that they said you can stay for your own observing, but you would need to verify that. Even if they allowed viewing at the top, you would need to bolt your scope to the ground and hope for the best because they said that high winds were a norm. Google "Onizuka Center Mauna Kea". They have a pretty informative site.
You might investigate Haleakala on Maui. It is at 10,000 feet and a National Park. The views up there were spectacular during the day!!
Before you go to the trouble of lugging your scope on a long trip, you should know that the visitor center at 9,000 feet has about 10 telescopes available for anyone to use, including a couple of 10" Dobs with Telrad finders. It is still really cold at that level, so figure on 40 degree F weather. If I were you, I'd save the scope space in your luggage for a warm jacket!
When I was up there, I was the only one using the free scopes. Most visitors are newbies who depend on the volunteer staff to point out stuff and observe though the club scopes that are permanently housed up there, including a rather large SCT.
The car rental places don't want you driving up there, that's why they say you need a wheeler. You don't. But it may void your insurance as provided by the agencies.
I drove up anyway, and was glad I did. Even after acclimating at the visitors' center, at the summit, when I stepped out of the car I was seeing lots of little twinkling lights, and it was early afternoon! There's not as much air up there.
Also, despite the altitude there can still be high cloudiness, and, as others have said, it can be quite windy and cold.
Weather permitting, I'd try the visitors' center, and there are lots of places beside the road below there where you could set up if it's more convenient.