Well at least it is a desgin problem and not a medical one (always be carefull with cheese graters!). I think the biggest problem would be that it will look burnt. So you should keep the end result still have a skin-like look. I would start by post-working/drawing on a new layer above a photo (or Poser-render) and try the different layer blend modes for the first dark layer, the actual 'tattood' part. Not just multiplying, but all the other blend modes. This way you can have the original light and dark shine through the dark parts. After that you could add a layer for depth and displacement, due to the scarring. Which makes me think you might look for some postwork tutorials concerning scars around here or somewhere else on the net. The layer blending modes help a lot. And look at reference photo's of tattoed people and maybe some medical sites, for scarring. It sounds uniquely interesting tough. If you can't get it going I would be glad to give it a try.