I get what he's thinking of - having the deck held inside by a softer, pliable material other than metal. But leather would be a little too "grippy" to be practical. In addition, you'd need to modify an existing Porper to have a wider gap or make a new clip designed for a larger deck in the first place.
I think a layer of something like microfiber would work better in this instance, especially if it's been "sueded" - by brushing the fiber with fine needles, you can break just enough of the fibers to give the surface a soft, suede-like feel and appearance with none of the disadvantages of actually using suede. I have a jacket from that material and it's incredibly soft.
HOWEVER...
Then you run into the issue of what do you do when the fabric, leather, whatever you choose as your inside layer, gets dirty and needs to be cleaned...
I think sticking to a plain metal or wood clip and keeping it clean works better.
Yap
If you stuck leather inside
It will hard to put deck in an original joe porper clip
I think the hard part is to find some where to buy leather
You can ask for leather store for tutorial 
But for emboesswd word on leather
It is another hard part 
