Thanks for the detailed reply, vadi92!
Following your advice, I did some more tests with other characters than the main one. The results for me, however, didn't change.
I now have tested both the windows 64 and 32 versions of the game in both native windows and emulation on linux as well as as smoe more tries on the native linux version. In all cases the button to change to the tattoo mode didn't appear for me, leaving me totally baffled.
And before someone asks, I tested against the save of my current linux playthrough as well as completely new saves. Granted for the latter I had to edit the save files, since I don't have the time for another full on playthrough right now. So my negative results could be because something broke when I changed the relevant values.
So it would seem that, for whatever reason, I'm an odd one out with all my computers. The only hope I can have is that some other players will toss their hats into the ring and add their experiences to the discussion, shedding some more light on the whole thing.
For competitions sake here is my version:
The loyalty status of my servant:
and her inventory screen:
And last but not least, the upgrade panel:
