Well, here's an intriguing development: My Skull Kid arrived today.
The intriguing bit is that I already got one weeks ago.
I assume it's because I purchased two 3DSes - one Ambassador, one XL - and one copy of Majora's Mask, and it's basing the skull kid distribution on each 3DS in turn. In which case I expect that the Kid I got with the XL was the kid for the XL, and then later they noticed that I was an Ambassador and had bought Majora's Mask, so sent out a further one for that.
So, in short, Ambassador ones might well be on the way.