I've been looking at how valve did it, it doesn't look very hard.
If I had the time I would make you an example.
Anyways, there already is an example in the source sdk content.
Go into the hl2 mapsrc and look for a map called "town" something, this is one of the ravenholm maps, and it includes a lot of climbing scenes.
Just look for one of those pipes and you should be able to figure it out.