I tried the alternate positions today and found the same results as Simon. The alternate standing position was far more stable and the alternate kneeling was about the same as the normal kneeling position.
I personally have no problem with genuinely disabled people using these positions.
However, I would like to point out that there is a big difference between the photo of the alternate standing position on page 2 of this thread and the official one on the BFTA site.
Basically, if you can put your left elbow onto the top of your left thigh then you have a position which is as stable as sitting and that can't be fair on anyone.
Anschutz 2013 MEC1 .22LR
Miroku 3800 12bore