Oooooohkay.. that's basically what threw me off, cuz when I read harness, it never really popped into my mind, the shoulder ones.. I just immediately thought waist harness for ammo and guns and stuff. XD
The shoulder harness should be alot easier. lol. Anyway.. after looking at it.. basically, I'd suggest trying to figure out exactly the measurements and look so it will look good, then actually sew it to the shirt, if you have that option, then.. again, only if it's possible, the front strap, I would sew to the inside of the shirt, just far enough that it overlaps(or would it be underlaps in this case? XD) with the harness on the outside, and sew it all the way through, so the leather on the outside and on the inside are sewn together. Should prove to be sturdy.. not sure how comfortable it'd be though. XD
But, being that the harness seems to go right up to the collar/edge where the shirt folds out, it shouldn't be hard to just put the strap right across there, so it'll go about an inch or two beyond that same folded point of the shirt, though, underneath.
I hope that makes sense.. just an idea, unfortunately I don't exactly have the pattern for the harness and whatnot.
Otherwise, I'd suggest just figuring out the pattern for the harness, making it entirely separate so you can try it on, make sure it fits just right, then make some slits in the shirt, somewhere in the area where the folded over collar/front is able to cover, then feed the straps through there.
But generally, I think my first idea would look alot better.