I've been working on this for a while now, inspired by the efforts of other forum members. I was only able to complete it after SC added the 'entityfloorlower' and 'entityislower' commands to the 1.19 betas.
It's a working ladder, which the player has full manual control over. To achieve this, I adapted rolfy & Xplosys' manual lift scripts (credit where credit is due).
After it's setup, the player approaches the ladder, 'uses' it (thus spawning an invisible lift at his/her feet), then presses the 'Page Up' and 'Page Down' keys to go up and down respectively. The player only goes up or down the ladder, for as long as the up and down keys are held down.
I made 2 versions of the ladder script; one for when the player starts at the bottom, and the other for when the player starts at the top.
In the download, I've included everything you need to set up the ladders, including an example map and instructions. The only thing I haven't included is a ladder entity, as there is already plenty available to download on the forums.
[edit]I think I might have put this in the wrong place, even though it includes an invisible lift entity[/edit]
[edit2]This needs FPSC 1.19 b6 or later, due to using the new 'entityislower' condition in the scripts[/edit2]
[edit3]Sorry, I just realised that when you open the map up, you'll get the 'missing media' warning come up, because I didn't include the ladder entity I used. You can, however, still use whichever lader entity you choose[/edit3]