Interesting idea, but I wouldn't grant any beaver this ability. My idea would be a new, specialized building with climbers. These beavers could be used to build (and man) temporary ropes that other beavers could use to overcome height differences. While only one beaver could use this installation at a time, it could be used to reach places blocked by trees, for example. So, this wouldn't be intended for regular transport, but rather as support for building (or farming/forresting) in difficult terrain.