Pounder, that sounds awsome. That or depending on how far you are going just have it where you are not straped in. The ones we always used as kids (homemade) just had a handle you hung onto and make sure you dont let go until you are at the end. Either way. Sounds like a fun thing to do.
Also, the girl that got injured was like wvbream said. She was on a homemade zip line, it faild and she landed in a river, cut her self, and ended up with a flesh eating batcieria. That is why she had to have limbs amputated. So I would not worry to much about that
I did see the south park episode. Yeah I am sure at a commercial place you be kinda like that, but then again aren't most commercial place for anything (rafting, caving, etc.) like that. Try to make it more exciting and like an adventure, that is how they make their money.