They show up as a gratuitous, white square which turns creating damage points for your fighter into a game of pin the tail on the donkey.
Unless, of course, you create a separate _damaged.dds (square) texture which does show up when selecting damage in the graphic selection buttons.
Did this with one of my fighters (see below):
To me, it looks like the editor is taking the information in the hull text file too literally and is actually looking for a fighter_damaged.dds texture
while the game happily extrapolates from the _sprite.dds.
Any thoughts? Conspiracy theories?