Well, it looks like using the sub image method isn't going to work either.
The problem now is the game uses the same image for two different classes, and it does this twice.
The S-18 and S-42 both use one of the five S-class images, apparently depending upon the paint scheme and conning tower configuration. The Sargo uses the same three images as the Salmon class.
I tried putting the S-42 info in one of the images, then edited the NSS_S42.upc file here:
MenuSilhouetteTextureName= S_Class_02b.tga
...to open it instead of the image listed. I used the 02b image for the S-42 info, the 01b image (the default) for the S-18 info. But when I tested it, it used the S-18 image for the S-42. Obviously this isn't the file that needs editing.
So I have no idea what to do next. While it works fine for the other classes, I don't want to have some with info and some without. And since there were some significant differences, I don't want to use one class and not the other. If it can't be done right, there's no point in doing it.