I have seen some of his videos and looked at the web store- looks like interesting stuff but haven't tried any of it yet. No idea about the laws governing them either. I have been interested in trying arrows from a sling but have been busy enough with other things that I haven't tried it yet.
Pocket Predator sells an inexpensive plate that is supposed to mount on just about any wrist rocket- I actually bought two of these but haven't gotten around to trying them yet.
Simpleshot also sells a system and bands for launching arrows out of slings. I think there are a few youtube videos out there showing that system, maybe including one from Fowler if memory serves. I think they even put it over a chrony and got a fairly reasonable 140 fps or something like that (just a tad below what an average bow would do), although it is hard to make much out of that since the weight of the arrow was not specified (on occasion, when it matters, I am a details guy).
I would love to know how it goes if you try a Chief AJ rig out. Like I said, something I have been really curious about. Just haven't found enough hours in a day or week to get around to it yet around all the fun I am having with building real bows and practicing with the slings so far. Someday though....