The collar is attached for our costuming purposes, with velcro under the collar which connects with the top of the back armor. In the MoM exhibit, I think it was that way, or bolted to the back armor. TylerDurden probably have the answer to this.
As of the picture you see in this thread, the little piece you see with bolts on it, that was an attachment part used for the pre-production version, that probably had a hook or something that was filed or cutted out as you can see in the pic, that later they decided to do it more simpler just bolting the cape to one of the studs he has in the collar from the top. The cape has a grommet so it's easier to screw in and out (that's the way we use it).
Robert