There are some English reel mowers that have used centrifugal clutches for a long time: it is not a new idea. There are both positive and negative aspects of using them.
The positive feature is that whenever the engine is at idle, the reel does not rotate - no operator-action is required. This sometimes offers a safety benefit, such as when somebody drops something or does something really dumb - which most of us do at least occasionally.
The negative features of a centrifugal clutch are also safety-related. First, if the engine speeds up without the operator intending it, the reel starts and runs. Second, if some minor mechanical glitch prevents the engine from slowing down or stopping, the operator cannot prevent the reel from running. A skilled operator will then pull off the spark plug lead, but not everyone has the knowledge or presence of mind in an emergency to do that.
I doubt that it is difficult to fit a centrifugal clutch if you want to do so. Remember though, you need one that operates at the right speed: above the Briggs' idle speed of 1,750 rpm, and below the lowest speed you would ever want to use for mowing, which is probably around 2,200 rpm. (The only other engine you would be likely to fit is a Honda GX120, which idles at 1,400 rpm.)
