Steve,
I'm fairly familiar with that canyon. I've participated in a couple of canyon cleanups with the Canyoneering Association of America (CAC) over the last several months. The CAC had a special use permit to go into the closed area. Going up above the base of the falls is prohibited as is climbing up the walls of the canyon. If anyone was out there canyoneering, they were doing so illegally.
Now, that said, all the signs are at the lower end of the canyon. There is a dirt road that goes up and around to the east of the canyon. From that road, there are several points at which one could drop in above the closed section and then work your way down stream. I highly doubt any signs are up above the closed section. The drop ins are generally technical in nature.
The closure is intended to stop people from climbing up from below which is the way that a lot of taggers, jumpers (into the pools below the falls), etc. were doing it. I don't think anyone thought of technical canyoneers descending the canyon.
So, it might be possible for someone to be in the canyoneering section without realizing that a closure was in effect, particularly if they weren't a local (it's been in all the papers and on TV locally).
HJ
