Hi Jim,
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!!