It's probably worth asking the BFTA directly, but when I queried this years ago I was told by the then chairman that this applied to the pad, not the hamster (my emphasis below)
BFTA Main shoot rules 2012 issue 3: Rule 10 Shooting : iii, d
The surface of the pad on which the stock rests must be flat and not shaped to provide lateral support to the stock
Give shaun shore a tickle, i'm sure he'll clarify.
As far as I know there's been one person using one last season... can't remember if he did any GP's with it last year, think he did year before.