Please do not do this, it isn't worth the extra save file size cost. I would rather see you make a texture for the two separate possible directions and use the metadata to store the direction.
Here is how I would do it:
when deciding the icon for the face:
if nbt (metadata) = 0 > choose north/south icon
if nbt = 1 > choose east/west icon
when placing the block:
use the code above to tell if the player is facing north or south, if they are set the metadata to 0, otherwise set it to 1.
From what you have said so far, I would assume this block is strictly visual and has no intended functionality other than being a block.
There appears to be some code in the Minecraft source that handles icon face rotation, but I haven't played with that yet. maybe later, as I do see myself hitting a similar problem in the future.