The workers at the Ballard locks in Seattle used to use these because of some safety regs (for a short while). They all hated them because they jammed up all of the time.

Wish I knew the name of the mechanism they used, it looked interesting enough to get me talking to one of the workers about it.

Edit- the best I can find is either something called a 'starpass glider' mentioned on page 14 of http://s7d9.scene7.com/is/content/minesafetyappliances/Horiz...

or http://www.saferack.com/lifeline-cable-systems-fall-protecti...

The system I saw at the locks seemed like a better engineered system, from what I remember.