I once dried out a rabbit skin, with pure salt.
It went very stiff, I just rubbed, crushed, scrumpled, rolled it etc and it soon came soft and easy to handle.
Took ages, as I only did it occasionally, and the Pup got hold of it and chewed it up Was going to be a toy for her anyway....
I think you can also remove the Brain of the animal and rubb it into the pelt and it will soften it, something like that anyway.
Good topic this