I was wondering, would it be possible to introduce various tattoos compatible with the BB meshes and modyfying the chargen process to enable body selection in addition to choosing your character's face and hair?

The way I see it, crafting those as texture replacers would force said tattoos on every NPC (and character) of a given race. How about introducing them via a texture modyfying/replacing scipt? Any thoughts?

Integrate them into Armor/Clothing. So you also make sure, that it is seen while wearing something. This way you could also make them asymmetrical.

Or you make a special race: Breton | Tattooed, Orc | Tattooed, etc.

I'd love to see something like this implemented. I've seen both modified races and tattoos as clothing, but imho, both ways are flawed; as clothes, you tend to get weird conflicts if you use lots of clothes mods (you can wear tattoos, or pants, but not both) and as modified races, you have to manually put every face and hair style you like into the new race from whatever mods you use, which can take forever. I'd love to see a "body type" selection, if it were possible to script. Then, you could not only add tattoos, you could make older people, people with different weights, etc.

I would imagine, though, that you would have to use some sort of script extender. Not that I actually know anything about it.

I've thought about that. Lyssia, however, summed up my issues with the aforementioned methods quite accurately. That just won't cut it, not for me anyway.
One option of introducing tattoos AFTER the chargen process would be via a body texture swapping script available through an in-game tattoo artist/vendor.
To ensure compatibility, vanilla BB textures/Westy's hi-res packs would be optimal.
Perhaps a similiar dialog could be initiated during character creation; also - I believe this could be done without resorting to the MWSE by using a slightly "mutated" version of the Enablestatreviewmenu script. Still, I'll need to dig deeper to be sure.

As far as I know, swapping out a model's texture in game is impossible.

Sounds neat though.

I agree, I have never heard of this being possible, even using MWSE.

Could you set the new body up as a separate mesh and texture, so there's no swapping involved, and just use the script to point to one model instead of another? Similar to the way faces and hairs are set up, for instance. You pop up a window, and select from models that might have identical meshes (ie, all the standard Better Bodies mesh), but that are named differently to make them different objects, and that have different textures associated with them. Then it's just a matter of figuring out how to create the window, and get the game to let you select the body model as well as the face and hair models, and keep that unique body model/texture associated with the player.