My brother suggested this, and we tried it out (a few times) while in arizona. They were so, so good!
1. We started with fresh picked ruby grapefruit, and loosened the wedges by cutting around the membrane with a sharp paring knife.
2. We found that dabbing the top of the grapefruit with a clean cloth or paper towel helped, otherwise the sugar was too juicy and it wouldn't caramelize properly. Then add the brown sugar in an even layer across the fruit. We ended up with a hard candy top on one of our early attempts, which was certainly delicious--but not exactly what we were looking for.
3. Torch it--the sugar takes less than a minute to caramelize.
4. Enjoy!