That's a terrible idea that would simply overcomplicate things.
Minecraft wise it would be somewhat complicated but you could use separate files as well. However, then you have 4 files for just a face...but you only have to edit the front if you wish, so it's the same amount of work either way, it really depends on the motive of the creator. I'm sure there would be plenty of blocklanders ready to make some cool new decal ideas. I like that UV unwrapping idea =P
Newest update finished backside
Ninja: I thought Demian said he could script it up. But idk about editing the player model