yitayningwut,
You have forced me to revise my understanding of the Shach, and the halacha in general. Thank you. I was of the opinion that no one held like Rabbi Shimon. It now seems to me that the Shach could be holding like Rabbi Shimon, that a tattoo is prohibited only for A”Z. I also saw the B”Y who mentions that there are some that hold like R’ Shimon and some hold like the Chachamim. So apparently, it’s not a slam dunk as to which Tanna we follow.
On the other hand, your avenue of heter seems difficult to me because no one explicitly says a rule that you implied. No one says that in any other circumstance where it’s clearly not for A”Z then it’s mutar. The Shach implies that, but does not say explicitly.
The Tur starts off by saying that it’s Chukai Hagoyim (sounds like Rabbi Shimon), but also says one is Chayav even if he does not write a name of A”Z (sounds like Chachamim). And the B”Y says that even according to (Rashi’s explanation of) Rabbi Shimon a tattoo without a Shem A”Z is still assur (but no Chiyuv Malkot).
Tur: http://www.hebrewbooks.org/pdfpager.aspx?req=14272&st=&pgnum=77
Why then is the case of Efer Mikla mutar if tattoos are prohibited by all? I feel forced to say that there is another reason, like health or that it’s incidental. I do realize that no one expresses another reason, but at the same time, no one says that one can put ink into the open wound, and the wound would be mochiach why it was there. There seems to be something unique about Efer Mikla in a wound. I don’t know what or why, but the poskim I mentioned don’t go any further in their heterim.
If you could shed some more light on the subject, I would greatly appreciate it.