My own, totally unofficial, take on a kneeler is that:
3 points of contact should be allowed: leading foot flat on ground, opposite knee in contact with ground and opposite foot in contact with ground. I believe the sole of the opposite foot should be vertical with the toe area only in contact with the ground. I do not believe the top of the opposite foot should be flat on the ground allowing the sole of the foot to be horizontal and so act as a seat when a bean bag is used to support the area between top of foot and ankle/shin. Note that it is quite clear and already forbidden that the 'outside' ankle of the opposite foot cannot be flat on the ground to allow the shooter to sit on the 'inside' ankle and side of the foot.
I stress that this is only my opinion and although I see many people shooting with a bean bag support and sitting on the sole of the opposite foot I do not believe this position is forbidden under current HFT rules. It does, however, seem to provide a more stable position than the one I have described and allow the kneeling position to become a 2 point of contact when a bean bag is used, viz: the leading foot with sole in contact with the ground as contact point 1 and then the opposite knee, shin and top of foot coupled with the bean bag all acting as a convenient seat in contact with the ground as contact point 2.
Whether it provides a competitive advantage to those who use this 2 point contact position I do not know.
Personally it does not really bother me.
Now retreats to await the flack