Hi runningdeer,
I also searched this issue on these boards and someone wrote that you can use the ..._O01.dds files to add numbers facing in the desired way but those files don't do any actual texturing but only show the areas to be textures (or something like this).
So you basically only can add white numbers because you only can leave out parts of those files by marking them with your number and those parts will be left untextured and will appear white in the game (or was it black?). So they are only really useful when the contrast to the rest of the hull is good enough.
I think it is okay when you use it for numbers as those aren't that detailed anyway. I didn't try it myself because soon after I read this I decided to go with a emblem and paint scheme only
EDIT: Here's a skin that actually uses numbers on the bow of the sub and they even show up correctly in the game. You might use this for comparison.
http://www.subsim.com/radioroom/showthread.php?t=133038