Trees .. the ones I saw, no. There's one incident I attribute to BF which has puzzled me ... up a tree is a very logical possibility.
There's a report from Northern California, either Redwoods or Jedidiah Smith park, of a big redwood back toward the edge of a park area which has bark that suggests signs of something climbing the trunk which, if I remember right, is bare for 60 feet or so but impossible to see on up. I haven't been back to the area since I heard about it so I haven't taken time to check on it myself. Sounds like a good spot for a trail cam.
The two I saw were essentially at the same level I was, very slight slopes only. Your idea is not bad though.
MIB