WHY NOT ?
I have seen all sorts of items used as cleaning / polishing media. Even saw crushed up peanut shells used in a rotary tumbler, forget the use right now.
If one could build a cheap "Ball Mill " you could crush anything like that to any size you wanted it. A simple ball mill could be any rotary tumbler with steel balls of unknown size to me, but shouldn't be hard to find on Google..!! ( Look it up )
I do not have pecan trees any more since I moved 15 or so years ago..Use to have about 12 LARGE ones in Houston..!!!
Had many bags of pecans...also used shells for the BBQ and wood also...it adds a very mild flavor if done right.
I feel walnut and pecan shells are about the same hardness...walnut may be harder. But should work fine for media if crushed like commercial walnut media.
I do not have first hand info.