Yeah, that's one way and probably the easiest. The other one is to go into the .map file and change the filename of the referenced map picture to the muted one. That was a terrible explanation, but if you open the .map file with notepad or some other text editor you should be able to see the filename you need to change at the beginning of the file.
Hmm... I really like the pretender I have set up. Actually I'm a pretty big fan of Midgard in general. I've stayed away from the norse-inspired nations thus far for some reason (I think the giants scared me off), but that may have been a mistake. That's not saying I think I could actually win or that I'm any good with them necessarily, but it's a fun nation/pretender combo. Also, this map is a little odd and the cave systems are going to be an interesting twist.