While not available for trailers, I can vouch for their function. I had the left rear tire go on our Z4 last summer on dry pavement and a straight section of road at about 50 mph. Since the top was down, I heard it first, then the dash indicator lit, and I felt a very minor momentary drift. I slowed down, drove home, order a new set and drove it in for replacement. All very civilized.
The challenge was finding a shop that had the equipment to mount run flats. Apparently the heavy side walls require special tooling. They are expensive compared to standard tires and, if they ever become available for trailers, you'd want a pressure monitoring system, or you may not know if one failed - even on a single axle trailer.
Roy and Marie