My boa is only 4 feet right now, I've heard that the larger they are, the less arboreal they get. Not saying it doesn't happen, but I don't think she'll be on the tree often for feeding as she gets older. As for feeding her while on the branch, if she's anything like my boy, once she smells and sees the rat she'll probably creep off the tree and get into that "I'm gonna eat you" pose. Just a guess, my boy doesn't have a tree lol.

As for the aspen, I have my boa on aspen as well and I don't put paper towel down for any of them. Mine eats f/t and it doesn't even get a chance to touch the ground before he strikes at it. Plus, ingesting a little aspen can't hurt. I have had to pull a couple small pieces of aspen out of his mouth a couple days after eating though. If he can't remove it himself I just hold his head and use tweezers to remove it, it's annoying but don't think it will harm them at all. Hope I helped some!