They are legal since 29ers became legal for racing. Not sure it's a good idea though. Last year a few racers decided to try their cross bikes at our provincial championship XC race. The course consisted of 5-6 technical areas, mostly descents with one technical climb, linked by easy trails. Some of these crossers were excelent athletes who would have been competing for the podium, all of them finished well down of where they likely would have on their xc rigs.
I also tried out my Jake the Snake at our local short track XC course the other day. The course has 0 technical sections, but is bumpy in places, I was much faster on my xc bike.
To me the big advantage of a cross bike is on smooth untechnical trails or on grass. The difference between 2.0 race tires and 35mm cross tires in grass is night and day.