Just checked my Greene's "The Warplanes of the Third Reich" and he indicates that the Me 321 wing had a wooden leading edge and the rest of the wing was fabric covered. Which is somewhat deceptive because the wing is built including the wooden covered leading edge back to the first spar and then the entire wing is covered with fabric (some ragwings use metal to cover the leading edge). So in theory at least, there would be no lines other than between the wings and the control surfaces.
Now if I may, I'm going to rant a little about something which I am somewhat familiar with in 1 to 1 scale. I tend to get real heart burn in how the kit producers define the fabric covered surfaces on their models. I have no problems with the sag they build into them (thats what fabric tends to do under most situations), but their rendition of the cloth "weave" is ridiculous. Take a look at the sheets on your bed. The weave is somewhat hard to see even in 1 to 1 scale, let alone reduced to 1/48 or 1/72 scale. And this is before brush applying 3 coats of clear dope and 3 coats of Aluminum dope (minimum required by regs). In otherwords, on a model the only thing to represent the fabric covered areas should be the sag in that particular area.
I'm always amazed to go to an airshow and hear a number of people state that thay didn't know that the rudder of a P-51 or P-40 was fabric. And along the same lines, all of you know that the Mosquito was a wooden aircraft. but I'll bet the majority of you don't know that is was also fabric covered. Yep, they built it from composite wood and then covered it. Did several things for the airframe. Added strength, was much smoother and lighter than just paint or varnish (once doped and sanded) and helped retard rotting. There were several civilian aircraft built the same way (Bellanca aircraft come to mind).
So when treating an item which is supposed to be fabric covered, I try to remove the "weave" the kit manufacturer has applied. Thanks for letting me rant.