Even if it is legal, it shouldn't be. Too many Kinect's on the field could definitely cause too much interference and unexpected results. The Kinect operates on a 830nm wavelength (near the wavelength of a laser mouse) uniformly across ALL Kinect's manufactured so therefore if at any point during the match if the 830nm projections are projected onto other projections or if projections are projected on top of other projections there will be interference. The more Kinect's in one area, the higher the chance of interference. There are also other things to worry about such as IR-absorbent material on the field and such, not even to mention the difficulties you will have in getting the Kinect to work through the cRIO.
All the information you will ever want:
http://openkinect.org/wiki/Hardware_info