Here's a hook our FTC team (TNT 8372) is using to do the pull up on the mountain.
It can also act as an advancer. However we need to post on the FTC forum to find out if that's officially ok. At the tournament we were at to day in Roseburg OR (11/07/15) The field judge said it was within the rules.
It has holes on the top to mount a servo to hit the all clear

Two versions
V.1 has a narrower inner dimension for grabbing the pull up bar
(1.2 inches)
V.2 has a wider inner dimension for grabbing the pullup bar, also has 2 more holes (1.6 inches)

Doesn't Matter

Doesn't Matter

200 microns

75%

We slammed the hook into the ground may times and it didn't get damaged at 50% infill