Here's what I could find on a quick search. Not quite what I was looking for but you get the idea.
Reunion-of-Giants-Lancaster-Documentary-1 by Britt Vallot, on Flickr
You can make out how that lighter ash color defines the space around the rivets. I would also suggest keeping more of this color on the top sides of each little rectangle as it will give the appearance of light from above reflecting off this area. The darker color underneath will help suggest the panel skin receding.
Here's another example:
Lancaster-tail-turret by Britt Vallot, on Flickr
You can see the areas that receive more direct light from above will have that lighter color. Ultimately, they also tend to weather faster from the Sun as they receive more energy. So if you look just forward of the turret, you'll notice how the rivets "pull" the skin back into the ribbing and stringers creating a darker surface. I'd look to replicated that by keep your lighter weathering/painting on top the rectangles created by the riveting.
Also, if you can handle it...I'd encourage you to do some riveting =P I bet it'd be beautiful. =] ...albiet a huge PITA.
Looking forward to seeing your build!!