I also think that a trapdoor operated by a lifeguard might be somewhat "safer" than a DIY rocket launch as nobody can control your slide position or if you take loose objects (like jewelry or cameras ) with you.
On the other hand, normal speedslides with a high age limit also bear a risk if not used properly, and those slides are nearly never supervised by an operator who controls age and position. Additionally, you could use video surveillance at the top of the slide with a direct connection to the lifeguard's booth, and if you do something wrong, the lifeguard can stop the process immediately without being present at the slide start.
But as I already mentioned in the other thread, I suppose that the German TüV would be upset about such a regulation.
Rocket slides which are additionally equipped with a looping MUST be supervised permanently because if you get stuck at the deepest point, someone has to rescue you which can be guaranteed only if the slide is under permanent control. Look at the thousands of buttons the lifeguard at the AquaLoop in Cologne has at his control panel. It would be reckless to let someone slide on his own.
Concerning slides without a looping (X-Stream at Tikibad, Sloop at Nettebad Osnabrück), I could imagine that a DIY operation could be possible.