I kind of wonder if they consider that a skeg since it is swept rearward. Other skegs that I have seen are forward sweeping to really stop the plane. I would tend to think that the protrusion on the bottom will not break the plane any faster than landing on the tow hook would.
I am looking at the rules now and cannot see anything regarding a skeg.
http://www.modelaircraft.org/comp/05.../RCsoaring.pdf