Warren, I'm not a hammock user myself, but in my opinion, it would be quite a hassle to try to find a camping spot where a hammock would be accommodated.
As for rock crevices: Most "common" overnight places don't have boulders and rocks large enough to use for hammock hanging.
While a few common places have enough trees, I just wouldn't count on finding them every place I wanted to camp, as you would need to try to camp below the tree line.
However, I just did a search on the Facebook "John Muir Trail
" group and found this: Has anybody hiked the entire trail using a hammock?
and several reported they were able to hike the entire trail. I suggest you read that thread.