Now Jack you've hijacked my thread already
But I'll answer your question.
Having been at multiple meetings of the BFTA the cover all answer is "chief marshal". It never works.
If the kneeling definition can avoid ambiguity then it will marshal itself.
Originally the rule was about keeping the knee clean (the mat will do that and offer comfort from stones etc), offering ankle support (the ankle roll will do that).
The mat being flat cannot be abused (if it can then please write how).
The ankle roll can only support the ankle not any other part of the body.