I messed around with a first person controller and building a ladder. The first person controller was based off a Brackeyes video and I created the ladder off of that base code.
The image to the right is the Unity Editor view of the ladder. Its has two colliders attached to it. One collider is used for physical interaction with the ladder such as standing on top and preventing the player from running through it, while the other collider is for detecting when the player is in range to start climbing. The larger collider is the one that detects whether the player is about to start climbing.
Right now once you climb the ladder you can’t back up off the ladder, but I think that should be simple enough to do with a small change to the ground check system that is currently implemented.