That flag btw is only on the faces not the body parts ...
No, it's not. Have a look at the female bartender torso nif (meshes\clothes\barkeeper\f\torso_0.nif), for example: it has two tri-shapes, one for the dress that doesn't have the flag and one for the body that has the flag.
I've just tested the following method and it works:
- create a new texture based on the existing body texture (e.g. textures\actors\character\female\femalebody_1.dds)
- add a tattoo to it and save it using a different name (e.g. femalebody_1_tattoo.dds, there are 4 in total: male and female for both "normal" humanoid bodies and Argonians)
- open the CK
- find the TextureSet 'SkinBodyFemale_1' and edit it
- edit the ID to 'SkinBodyFemale_1_tattoo'
- replace the Diffuse map (the actual texture) with your new texture that has the tattoo
- close and create a new Form when prompted
- find the ArmorAddon for your armor (I tried 'BarKeeperTorso01')
- set the 'Skin Texture' of the ArmorAddon to your newly created TextureSet.
- save the mod and play

This worked for my very dark skinned Redguard with a black tattoo. Other more muted colors will probably be shifted in color by the color shifting the game does to create the different skin colors. Saturated colors will probably work best.
Texture:
[img]https://dl.dropbox.com/u/170850/Games/Skyrim/FemaleBody_1_tat.png[/img]
In game:
[img]https://dl.dropbox.com/u/170850/Games/Skyrim/tattoo.png[/img]
Of course, this makes the tattoo only appear with this specific armor/clothes.