>>Adding whither strap to western breast collar<< I've seen it done out here. A saddle maker can add "arms" from the breast collar + a whither strap encased in a removeable tube of soft material depending on the design of your existing western & the location of the front d's on your saddle. I'd take the saddle you wish to use it with to your local saddler. Karen's idea of also taking a template english hunt style breast plate in is great so everyone's on the same page. Just make sure you share that you prefer that all additions have skived "no feel" edges on the horse side junctions , all buckles are shielded & all material edges beveled. I also like all hardware to be as light weight as possible (snaps etc...) to reduce extra weight. I would also install in the no-break synthetic materials some kind of fail-safe in at least 2 of the key partr, kind of like the fuses you see on break away halters just in case something catches the breast plate. I want mine to break away clean . This is why I prefer all leather tack to biothane..just for saftey's sake.
Saddlers ideas of what is acceptable in terms of horse comfort vary widely as I have found in past, so be clear on what your needs are. Always best to learn from someone else's mistakes rather than your own!! Kaaren