It's actually just a color layer with effect/transparency (different setting for the different maps) and a background paper texture. I didn't used the same paper texture for the world maps and this map, and I had to "make it bigger" from the originals (which came from deviantart with a "free to use" policy). I can put it at your disposal if you want.
I also would like to take the chance to ask you about the sea in those gorgeous maps, did you have to make it yourself or is it one of those background images you can download and use directly? If it's the last case, could you perhaps direct me to it? I suck at creating nice looking textures for my crappy map trials, and that sea you got would solve my sea problems. If you can't don't worry.