If there is any predisposition towards an ankle/knee injury then the BFTA rule should be applied where the shooter can take a kneeler standing (or compete in Open Class and take them sitting).
Kneeling rolls are something else that is discussed but they need to be of different diameters to suit different size shooters and also different elevation/declination of the target.
I think this is all showing that a correctly held kneeling position is a challenging shot, possibly more so than standers which has, all the same, seen kneel:stand changed to 2:3 on GPs to split the top AAs. Maybe if done properly kneelers will split them and not the edge of the KZ :-)
All the best