Yep, I think I'll leave the diagonal as 1:1:1 and do the colour tweaking in post-processing.
With trial-and-error experimenting, I answered my question. The correct offset for the Sony NEX-3 was X=1, Y=1. So there you go!
I'm still not sure what Bayer pattern that corresponds too, given that I don't know what the default
Nebulosity expects, but that's all rather immaterial to me now
