Regarding the Mani mantra, from the Box Sutra, as reproduced in A Garland of Jewels
by Ju Mipham (emphasis mine):
The Bhagavat Shakyamuni once said, "It is valuable for beings to hold the name of Avalokita. His great awareness mantra of six syllables was sought for sixteen kalpas by all tathagatas. Even the great mother of tathagatas prostrates to this awareness mantra.
"Those who hold it and recite it will acquire immeasurable merit. At the time of it's recitation tathagatas and bodhisattvas equal in number to the smallest particles will gather. Millions of buddhas will enter the pores of the reciter of this mantra. They will bestow their approval, saying 'Child of family, you have well acquired something worthy of acquisition. Even all the beings who live in your belly will become irreversible bodhisattvas.' The reciter will be guarded by devas, nagas, yakshas, and others.
"Anyone who keeps this mantra on their body will achieve a vajra body and a buddha's wisdom. They will acquire all complete qualities, including confidence, wisdom, love, and the paramitas. They will quickly achieve the unsurpassable awakening of buddhahood.
"Any being who touches or sees this mantra will become a bodhisattva who has reached the end of rebirth. This great awareness mantra pulls out the root of samsara. It guides one to liberation and omniscience. In search of this mantra, one should fill all of Jambudvipa with the seven jewels and offer it. If someone wishes to write this mantra down but lacks ink, it would be excellent for them to use their blood as ink, their skin as paper, and their bones as a pen."
~ pg. 168
It is not lost on me, however, that phrases such as "Anyone who keeps this mantra on their body" could refer to wearing amulets and/or jewelry. And the part afterwards about using one's blood as ink, skin as paper, and bones as a pen is pretty obviously either meant for high level bodhisattvas (e.g. the Buddha cutting off his head and offering it from the Jataka), or as figurative language.
That said, it is hard not to see the quote above as scriptural precedent for Dharma body art, at least to some extent. This seems to be an issue that is being talked about more in recent years, but still there is no firm "yea" or "nay" from many teachers on the subject.
"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།