The reason they tell you not to get a tattoo post transplant is because how susceptible to infections and viruses. And there is a risk of getting your tattoo infected if you go to a place who doesn't take the proper care in cleaning their equipment. Also, keeping the tattoo clean while it's in the healing process adds more risk to getting an infection and increasing the possibility of your new organs adapting to your body properly.
I've read somewhere where people were getting tattoos about 4 years post TX and doing just fine if you take proper care of it, and go to a sterile place, which is easy to do. I don't think any doctor will tell you that you can get a tattoo post TX because of the risk of infection, but people have done it and been just fine. Just make sure that tattoo is worth it.