I've always been told this is not legal. But, the kneeling rules do not specifically ban it, and the kneeling rules do not specifically state that the behind must sit on the trailing heel.
"A. KNEELING: There shall be only 3 points of contact with the ground (2 feet and 1 knee). The rifle will be supported by both hands and one shoulder only. The arm supporting the rifle must rest on the knee or leg. The rear foot shall be upright and straight in line with the knee. A legal seat may be used to support the rear foot and/or ankle, provided that the foot and knee have contact with the ground. The leading hand shall support the gun, and be unsupported forward of the wrist joint."
One might submit this as one possible solution for medical exemptions in the kneeling lane. Some knee replacements do not permit more than the 120° required to sit on the heel. Not necessarily advocating this, but at least it permits the shooter to reasonably adopt a standard kneeling position, or the spirit of... (rather than sit upright on a bucket shooting chicken wing off-hand).