Yes to both, there will be piercings as well. Implementing slave customization is one of my top priorities and will be coming hopefully in 0.0.3 and at the latest in 0.0.4.
And good idea, when tattoos get added I'll make sure to include barcode designs as an option.
This would be interesting, but unfortunately it'll likely involve in-game texture generation, which could take some time to be done properly. While it's not a top priority at the moment, I do plan to add this in the future as it opens up a lot of possibilities, such as tattooing custom words on slaves and other cool stuff.
The solution you are describing would work perfectly for sprite-based characters. However, the problem with 3D models is that each texture needs specific UV coordinates for proper placement on the model. This would mean that I would need a texture for each number, in each position, for each tattoo design. Even if implemented like this, it would also mean that each number applied to a character would have it's own mesh with everything transparent but the number, which would hurt performance a lot.
I've been thinking about this recently, and I thought of a approach by making a custom shader that might just do the trick