Thanks, that works!
So the trick is shiptype_shipname_shp.dds, little 5KB file with a blank white rectangle image, the shape is actually in the alpha channel.
NDD_Fubuki_shp.dds N = Nippon, DD = Destroyer, Fubuki = name, each ship has it's own dedicated _shp.dds file so in theory you could change seaplane tenders to a different shape to distinguish them from actual carriers, or even make escort carriers more rectangular or whatever.
Never expected that, I was assuming a single image in menu\gui\layout or something, with all the shapes on one image and something in the configuration of each ship that takes it from a section of the image. This will be much easier to mod, time to play with making small medium and large for merchies and/or them evil culprits with deck guns.