Thanks for all the motivating comments

. Progress was slowed slightly in the last week or so, due to getting ready for the local IPMS show on the 16th, and starting another quick build with a deadline at the end of this month. But I'm still working on the mail plane too.
Here are the wings, with the lozenge decals and the overpainted German crosses. The lozenge is done in 5 separate strips per wing, and the edges of each strip have an irritating dark border. That necessitated patching every joint, which must be why WNW supplied an extra couple of lozenge panels. In this photo, the upper wing has not been patched, and the lower one's outer joints are patched. The W.12's lower wings are each one large lozenge decal, which sounds like a lot less work.

Although not visible in the two extant photos of the mail plane, it's assumed the wing crosses were overpainted. I chose to use Tamiya XF-19 Sky Gray, just to add another color to the plane. It could have been either of the fuselage colors too... or any color, for all we know.

The lower surface of the wings has been painted, ribbed, shaded and crossed out too.

I've read somewhere that there was a black leading edge tape, but I don't know if I'll attempt that or not.
Finally, I couldn't resist slipping the wings onto the spars... strictly for a fit test, of course!

Man, that's got huge wings! Presumably the wing area is the same as both wings of the W.12 combined, or more, since the W.29 was larger and heavier.
Peter