Jeff,
I broke my clavicle several years ago and landed on my right shoulder wearing a Fox cheast protector, the right shoulder cup looked like I tood a 5 lbs mallet to it! Fox sent me two new shoulder cups which snapped into place. I wear my CP under my jersey with a North Face micro absorbing shirt as a t-shirt, I also have a Nike Under Armor shirt as well. My helmet made contact with my cavicle also, you can see the dent in the black moulding just to the right and below the chin area. My doctor said that the break was more then like likey caused my rhe right shoulder being slammed to the ground and then the helmet contacting the bone, which was enough to break the clavicle...kinda like a chisle?? I now wear a donut to help pad that area. I'm no expert, but a chest protector will only do so much when that type of trama is placed on the shoulder area...my .02
I don't think I'd like to wear on of those EVS vest / chest protector and have a broken clavicle....that would be pretty painfull having to take that off...or have a doc bones take a shears and cut it off...