BB, you mentioned earlier you were even considering the Lock Jack. If you are referring to coming off the crane in a doubled rope setup, the Lock Jack will definitely do what you require. You can obtain free fall speed very easily if you run the top end of your rope through a pulley.
This is a safe repelling device in the fact that if you release pressure you will stop. There is no rope twist.
There is a learning curve. I would borrow somebody elses first to try it out. People either love em or hate em.
I use mine with Velocity and though I use many other hitches, such as VT with the hitch climber, and there is nothing that will have the same full release feeling of the Lock Jack and still be able to stop you with no effort.
If you use it in this manner, you will wear out cams fairly quickly. Kind of like a high performance car upkeep. It can do a lot of things, but it is going to cost you.
Dave