Hello Big D,
I'm not a tattoo artist, but I have been interested in tattooing leather also. A while back, there were a few posts on this forum that showed some tattooed leather work, and I asked questions then, in hopes of maybe buying a cheap tattoo machine (for leather only). I wasn't able to gather enough information on the techniques used, so my attention moved on to other projects.
Having worked with leather and leather dye, but never using any tattoo ink, I can only guess that the results of ink against bare leather would be similar. I wonder if you were to seal the surface of the leather with a water repellent sealer before tattooing, would that prevent the ink that pools on top of the skin during the tattoo process from staining the surface?
I would suggest that you try a series of experiments using sealed and unsealed veg tan and chrome tan leathers, different sealers (maybe even liquid latex). I would try the veg tan leather dry, oiled, lightly damp with water, and soaking wet.
I hope you find the right technique, and I hope you're willing to share the results of any experiments.
ken