I have been trying to determine the answer to the exact same problem. We used a winch on our 2008 robot that worked really well. Here is a video of the winch engaging. Look at about 2:20 mark for the winch engagement.
http://www.youtube.com/user/Broncobo...38/pIV7yEc4xD0
I have been looking for some good pictures of the winch but cannot find any. We basically used a dog drive with a pneumatic release however we had a latch that would engage the arm. If you look at the video and there is a part that looks like an upside down coat hanger. There was a bolt that stuck out of it with two bushings on it and a set of hooks drove over the bushings then the winch was shut off and the dog drive was dis-engaged. Then the lock was actuated by a 3/4" pneumatic cylinder. We were waay too concerned about the shock load back into the transmission from disengaging the winch roll under load. We were drawing nearly 150 lbs with that robot.
Our team have been kicking around an idea of using a very similar idea for this years shooters with a little less monkey motion that was in that winch. We are hoping that we can accomplish a simpler method of the winch this year and be able to vary the power. I will not give away the whole idea but think about how you could use a ratchet and pawl to accomplish this task. I think this will allow the load to be removed from the gearbox to hopefully limit the damage. (fingers crossed)