Opinions on tattoos within the Tibetan tradition varies as widely as the dialects of Tibetan language. As always, carefully examine your intention for getting it done. If you have a guru, I'd ask him or her before getting it (if you haven't already). If not, try to generate bodhicitta when it's emblazoned on your skin and, ideally, view it as a reminder to practice. All too soon the mind, the guest, will depart the guesthouse of the body. It's far more important to internalize the practice and qualities of Chenrezig, in this case.
That said, Tashi Mannox is one of the most skilled calligraphers of Tibetan I've ever seen, and has dedicated a good portion of his time and effort to ensuring the accuracy and integrity of tattoos done in the various Tibetan scripts.
Here are the pieces he's done of just the Mani, including several done vertically:http://store.tashimannox.com/catalog/result/id/0d3e370513de95c2f3221dc04c11f2862884dd55
"The Sutras, Tantras, and Philosophical Scriptures are great in number. However life is short, and intelligence is limited, so it's hard to cover them completely. You may know a lot, but if you don't put it into practice, it's like dying of thirst on the shore of a great lake. Likewise, it happens that a common corpse is found in the bed of a great scholar." ~ Karma Chagme དྲིན་ཆེན་རྗེ་བཙུན་བླ་མ་རཱ་ག་ཨ་སྱ་མཁྱེན་ནོ།