The best way to do this is paint in layers. One layer for each arm, AND each side of the arm (4 layers total). Then move the layer as needed. Note you will probably get some pretty bad distortion at the seams unless you are using very plain tattoos (i.e. tribal lines). Don't be surprised if you'll have to remap the part in question to get a really good tattoo to work. I'm not sure what your question is about Morph targets. I use them to reshape a face, or body part in a way the dials do not allow.