hmmm interesting question, I haven't come up against something like this yet. I bet if you ask in the ship forum you might get a quicker response because some people don't come into this part at all. I'll have to watch if you do post this there because I would like to see thair methods. One day I plan on doing a USS Texas (BB) in one of her pacific patterns...
here is something I found on how one guy did it, similar to how I mask hard edged RAF disruptive camo on aircraft.
http://www.steelnavy.com/FelixAjax.htm#Painting%20The%20Camouflage%20Scheme