Just a thought but how about this as a rule.
Everyone has to have a 4" circle on there bean bag,say in a bright colour e.g yellow or something so it can be clearly seen.
This can be made from cloth or something or even the bfta have 4" badges made up (chance of making some money from it) say charge everyone a fiver for it.
This is then placed at a determined distance from the edge of the seam/bean bag.
Then when taking the kneeling position, the knee must make contact with the patch.This would make it easy to police as anyone who places their knee forward of this point would clearly be seen.
Just a thought so don't all start taking the pi$$