I'm not a BFTA rule specialist by any means as I'm fairly new to FT.
I've read a lot though and asked questions. My understanding is that the sling issue isn't a particular bias against slings or their original intended use. They just get covered by a general umbrella rule, that basically allows any design of stock to be used ... or basically any gadget to be used ... but all things must remain in place, on the rifle, throughout the course and for every shot.
So, for instance, you can't wander around with a bag full of different hamsters or butthooks and keep changing these, to give an advantage, as you go around the course. People could also add and remove things from the stock to give better/different fit or weight for different positions like standing etc.
So you can't really get around that by saying that a certain item was on the rifle at the start and the finish ... but not all the time during. It has to be on ... and stay on ... for every shot.
However, you can adjust things on the stock etc. So people get around some of this by having multi adjustable hamsters etc.