I wouldn't want kneelers taken out of the comp's, that would be a great shame.
As for keeping your knee clean, firstly I don't care anymore and just get a dirty knee. But occasionally in the past (when the ground was wet) I have used just the thickness of the beanbag material under my knee and used the stuffing at the other end under my ankle giving me no added advantage except a clean knee!
Anschutz 2013 MEC1 .22LR
Miroku 3800 12bore