It seems Maui, Hawaii would be good for hammock hanging since the climate is warm and alot of the island is wet and forested. However, I was there last week for 7 days, with only 1 night in a hotel, and was only able to hang 1 night.
I did a dayhike and an overnight backpack trip in Haleakala National Park. The dayhike was on the Sliding Sands Trail down/back up from the summit. I camped 2 nights at Hosmer Grove Campground. This campground stinks. It's just a small, crowded, sloping grassy area that collects a lot of water from frequent rains. You're way too close to your neighbors. Since camping is limited to the small grassy area there's no good place to pitch a hammock. But, it's the only thing around. Atleast, there was water, covered picnic tables and clean NPS outhouses. And, it's free and the only place to camp in the area.
The backpack was out to Paliku backcountry camp on the Halemau'u Trail. Paliku was mostly windy and rainy but it did clear off for an hour or so. There's only 1 site at Paliku where you could pitch a hammock and it's close to the cabin. Unfortunately, it was occupied by a tent when I arrived. The hike out next day was soggy and windy.
Kipahulu Campground south of Hana on the wettest side is a very nice campground. It may be possible to hang just inside the treeline on a 2-3 sites but 2 were taken. Tree size and spacing was poor. People camped in the middle of the loop had a hammock hung on the multi-branched tree but I wouldn't count on getting that one campsite. They hung their lounging hammock among several branches of the large tree in the middle of the campground. There were mosquitoes and flies in the strip of forest so I decided to deal with the wind and tent it out in the open, grassy area.
Last 2 nights I camped at the private campground Camp Olowalu 6 miles south of Lahaina on Maui's west shore. Camp Olowalu's webpage here . I tented one night and a thorn tore a gash in my fancy new UL Therma Rest air mattress and tiny 1-person tent's floor. So, last night I was able to hang in the brushy coastal forest. Boy, was it more comfortable than sleeping on the ground!
Camp Olowalu is cheap at $10 a night. Highly recommended to rent a kayak and paddle out to the reef and go snorkeling. There's also a cool trail running north to the old sugar mill site on Olowalu Beach. Downside on this private campground is that you can always hear the traffic on the highway and the outhouses are inadequate for the number of campers. The showers were fine. There are permanent campers living at this campground, probably beach hobos and resort employees who can't afford a place to live. They were fine and didn't cause any trouble for me.
Pretty pictures of this week in Maui are at my Flickr set for Maui
I don't know about hanging or camping on other areas of Maui. There's state land called Polipoli southwest of Haleakala Park with a campground. May require 4WD access (?). Don't know about the West Maui Reserve lands either.