Have I tried a sheepskin strap cover? Yes. Both natural and synthetic.

The wool moves like roller bearings and reduces abrasion and absorbs sweat -- Very comfortable until it catches beggar-lice or grit. Then it wears holes in your skin. And it is hot. And heavy. And a lot of trouble to install properly. The only good use I found for it was on heavy packs - but I wised up and stopped packing anything heavy enough to justify using sheepskin. Slick straps seem to work just as well. If you insist on going this route you can experiment with synthetic sheep fleece to see if you like the idea. For long-term, I recommend natural fleece because it doesn't get as stinky as the synthetic.