Mh. I've searched a bit via google. Perhaps finally the skinner at MG are right after all. I found an image depicting a reconstruction of a 109G and it doesn't show the rivets and hardly the panel lines.

Please go to "Die neuesten Fotos" (see link list on the left).

The scroll down till you see the fotos of April 2011 and then click on the fotos where you have a medium close up of the back fuselage of the 109 G10

http://www.messerschmitt-bf109.de/

Of course with usage and dirt collecting they (the pannel lines) will perhaps get more visible like one can see on some old photographs. I never saw a historic foto with rivets clearly sticking out (they were anyhow sunk in the metal sheets).

Very detailled fotos of quite a rugged 109:
http://www.clubhyper.com/reference/bf109detailbg_2.htm

Here is another museum 109