The risk, in fact is not only associated with the trapeze, all sailboats have plenty of ropes and other stuff to get stucked with. I know of somebody that got trapped on a capsized Laser, with the tiller inside his lifevest, he managed to unmount the tiller, but recognized that it wasn´t easy to remain calm and figure out what to do.
I think it´s better that each one carries his own kinfe instead of storing it in somewhere in the boat where it can be out of reach. In the case I mentioned, the first thing the crew did after capsizing was to swim to recover the daggerboard that came out of the boat. She didn´t even think his partner was trapped down the boat and it took her a couple of minutes to get back to the boat.