Same problem. I tried to explain it here
http://forum.corona-renderer.com/index.php/topic,1992.0.htmlI don't understand in programming so it's may be funny for you. But if you see "v" near world-scale, may be it is possible to show in material slots bitmap 1x1 unit anyway?
1x1 unit material same for both kinds of mapping.
Let's say my diffuse bitmap AxB units. And Bump for example CxD.
So in materials slot I'll see diffuse 1x1, bump C/A x D/B and so on...
In case that we need only color for diffuse but have the maps in another slots, so A=C, B=D.
Something like this....
Sorry for my rich english :(