Had a stroke of luck and found a bunch of them at a Toys R' Us. I think all of them are compatible.
3DS Metallic Red: CEM 11646
3DS XL Red/Black Combo: SEH10013490
3DS XL Silver/Black Combo: SEF101807xx (two copies)
3DS XL Blue/Black Combo: SEF10187235
It does weird me out to find so many at the same place, big store no less. It might just be old boxes, but I think they'll let me at least check the back of a 3DS to see if it matches, and maybe even test it out if I ask nicely. At the very least, the metallic red should be a sure bet, but I don't find it in the European sheet, so can anyone confirm that?