Having searched the forums and even going on a long winded and difficult search of the issue tracker (who designed that nightmare) I have noticed that in the past there have been numerous requests for extra layers on the avatar for clothing/tattoos etc.
In particular a layer for the head has been requested and discused several times. I did notice a reply form a Linden from back in 2008 stating they were looking into it AND THEY WOULD KEEP US UPDATED (no further entries found).
Surely to add an extra layer to the head would not be too difficult as they have already got 2 layers for the other parts of the avatar. Think of all the new content that this could generate.
No more hunting round for a skin creator who is willing to add tattoos to one of their skins or a certain style of makeup. Also super heroes could have face hugging masks on a quality skin rather than relying on the generic skin.
I realise I am probably whispering into a hurricaine but maybe, just maybe a linden will read this and investigate the possibility of implementing this small but significant upgrade.
I am sure it would be far easier to implement than integrating SL profiles with Facebook and Twitter profiles.