Sorry, but I did not found time to check your solution earlier.

I think, I understood how to prepare: I load a G3 female as object into BS3D, apply to the torso the torso map, load a G3 female genitalia which is partly inside and partly outside of the torso. To this object I assign a blank map.

Then I shall 'Paint with a plain color around gen obj on g3f in order to get a guide for your painting'. Here I have the first problem: The Gen-Object is partly over and under the torso, so I do not see exactly where the gen starts. Is there an option to make the torso invisible to see the border of the genitalia? So I have no chance to mark an point outside the genitalia because I don't know, if the torso or the genitalia is currently on top.

'Select your genital obj and move it slightly horizontally.' Does this 'slightly' means entirely outside of the torso? If I move it only slightly and use the clone brush to clone that area, the torso get white where the genitalia is on top and the genitalia get only a texture when it's on top. Isn't there a chance to set the torso as source 'layer' and the genitalia as destination 'layer', means, when I activated the genitalia layer, I only modify it no matter what is in foreground?