I would think that as long as you updated it at least as often as you update your driver's licence, it would remain a valid indicator of your intent.
Why? Doctors have procedures for how to deal with organ donations. That procedure means looking at driver's license and organ donor cards. There are huge legal risks for them being creative.
I had to give up on trying to find out if a tattoo can count as consent on its own -- I would guess that it would be iffy territory unless you had it notarized and witnessed.
It might still be worthwhile to have a tattoo; it does tell them that you have given consent, meaning that they will make an extra effort to look for consent (In the US this means a state database). This would only be relevant if you are found without your drivers license/ID. There are a number of fringe cases where you might be found dead and dying without easy access to your ID, but...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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