You could try the 'second skin' technique.  Use the grouping

tool to pick out the facets where you want the tattoo, then

hit 'create prop' to make a separate little piece with the

exact same shape as the original arm.  You can then

treat this piece like a clothing item.  (Sort of like a glove

or hat.)  UVmap it, give it a transparency and texture, and

smart-prop it to the arm.

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