An interesting conundrum. Thinking about the book of earth (In the setting, not the supplement), we have excellent quotes to base this tattoo off of:
“That which has happened is gone. Indeed, it never existed. Whoever you were in the past only matters if that is who you are now. Nor has the future ever come to pass; it has always remained beyond the horizon. Who you will be you cannot know. Focus instead on who you are now. It is the only thing that is real.” (This is surprisingly apt for the character, given his incorporeal state. Perhaps the tattoo allows him to ignore attempts to change him, which mechanically means that ads/disads and spell effects imposed upon him simply don't have an effect, as long as they change who he is? Obviously it wouldn't negate damage spells, since the character being capable of no-selling a fury of osano-wo would be broken beyond belief.)
“The earth is eternal. Even as a hundred thousand beings trample upon its back, the land does as it always has. No matter how the wind howls, the mountain does not move.” (Very obvious, and usually the go to basis for earth related abilities).
“You can only lose that to which you are clinging.” (This one is interesting. If interpreted in a very literal fashion, as earth is want to do, perhaps it's impossible to wrest objects away from the character while he has the tattoo active? As a shugenja, I doubt he's the type to wield weapons unless he's a Tamori, so it's less about being un-disarmable and more about being impossible to steal from. Alternatively, the character can forgo any and all attachments when the tattoo is active, losing all social ads/disads and skills while the tattoo is active (Possibly material?), but gaining the incredible ability to spend as many void points on a single action as they want? I swear I had an idea there, but I lost it right at the end.)
“Do not depend upon partial feelings. Feel the ground beneath you? It has always been there. Many trample on its back, yet none dispute its strength. The earth holds no doubts and fears no force. Strike as the earth, and you cannot fail.” (Just in case they ARE a Tamori, or happen to be a martial shugenja, perhaps they can spend a turn in full defense stance to allow any one attack to auto-succeed?)
Just some thoughts.