My guess is that since EXO is touting all of the metal parts that went into the TWOK uniform (I know this was mostly to do with the Regula One jacket, but one can hope) that the shoulder clasp may be functional (at least in some way, unlikely that it would work like the actual clasp on the 1:1 uniforms being so tiny).
I think the larger question is, will that uniform flap fold down and STAY down for the classic look everyone wants. I'd say that the chances of that are slim to none. The fabric looks far too thick and firm for that to work. There would have to be some sort of a Velcro or catch to hold the flap in place after you unclipped it from the shoulder for any chance of this working at all.
But the biggest piece of evidence that this open flap look is unlikely to be possible is that this option is not highlighted at all in any of the promo materials. I this were a feature of the uniform, they would definitely be showing it off. I at least hope that the uniform material is the proper color by division on the inside so that brave customizers (in other words not me) might at least have a chance of making this work.
But hey, there's always PhotoShop as a last resort