In all honesty, I would talk to my local saddle maker. I have never seen the D-rings with leather being sold commercially so it would likely be easier to have them made. As far as putting them on without tearing up your saddle, I can't tell. Depending on how the concho is attached and how everything is attached under it. On my old saddles, the concho had a screw back that you just had to turn the concho and unscrew it to take it off. However, others are held on with glued nails that are nearly impossible to get out without tearing everything up.
I would worry about the durability too because it wouldn't be much holding the D-ring to the saddle, maybe just a screw or a couple of nails. I don't like the idea of hooking the breast collar to something that might break.