Aha, got it - the problem was that there's some layer-jiggling you need to do to get the transparency right. Namely, set a white background, take a mask and save that to the alpha channel, then delete the mask and swap the white background for a dark grey one to kill fringiness, before exporting as TGA. Mud, clear as :)

Thanks for the assist. The transparency is now working, but I've still got to work out the correct stretches and deformations to get the tattoo looking right: like most game skins the texture gets warped in highly-3D areas, so the belly "carpe diem" text is easy, but around the shoulders with this backpiece is trickier.

It's a very weird feeling putting a real-world tat on an avatar.