2 cents about skins...
I just tried to alter an existing skin.
For this test, I just made minor changes, like some more beard/chest hair, and two tattoos.
The major problem is, that you don't really see what you are doing.
Let's have a look on the tattoo on the upper arm:
Finding out, where to place it, is a "try and error game". The whole arm in the skin file is very short. So you have to keep in mind, that the skin is stretched and everything you draw on it is also stretched. If you alter the measures of the upper arm (diam/lengh), you get the same problem. After posing the figure in Blender et.al. there might be some similar problems.
Therefore for some kinds of modifications of the skin, like such tattoos, it is impossible to make a skin, that fits to all kind of characters. It's of cause no problem for general changes like colors etc., that has effects on large body parts.
It mainly depends how much the skin is stretched generally and by the different modifiers in MH.
I think, the only way to get good results is, first generating a character and then altering the skin only for this particular character.
Of cause, it's possible to draw on a final rendered picture, but as we have skins in MH, imho it's a better solution to do as much as possible on the skin image.