This is more just for curiosity rather than actually wanting to get one, but is there a place where I could buy a printed version of the CIE xy colour space diagram which accurately recreates all of the visible colours outside of the regular printing gamuts?
Its not actually possible to do. Because that paper you print on needs to have properties not avalialbe. The color on the edge are very pure and can not be mixed. I dont also see any way they could be reflected either. Also the xy diagram is not the full color space just a slice of it.
I think there is no point pursuing "objective accuracy" for colors, because individual perception of colors is different. The current standard is merely a statistical result for human eye perception. That is to say, it is possible for someone else to perceive the colors you regard identical as different.