as Mr Croucher points out the word hamster does not exist in the BFTA Main Shoot Rules
This is rule 10 part IV
Fully adjustable Rifle Stocks are permitted to accommodate various shooting styles and positions with no limitation as to design. Additional attachments are not permitted to be added or removed during the course of the event
Further parts of rule 10 refer to padded clothing
The surface of the pad on which the stock rests must be flat and not shaped to provide lateral support to the stock.
It would be illegal if your knee pad was shaped into a V that offered lateral support to the stock.
Your addition becomes part of the stock and - there is no limitation to design.
It's only called a hamster because it comes off. Rifles with deep forends don't have hamsters
Hope this helps