I will try to chime in with my two cents. We have the same discussion and here is what we did.
Definitely a grey area on the rules. But let’s try to look at the big picture, at what the original writers of the rules tried to do.
If you do not specify the prone position and let people shoot resting the gun directly on the ground it is EASIER that the “classical” sitting FT position, so they limited it with the definition of the prone position. But a “modified” or “illegal” kneeling position is no better in general that the sitting FT position, so we accepted it on freestyle lanes.