I think I have settled on using the following code to produce a frame size of 2048 x 1152.

########## anti-alias & upscale
NNEDI2(dh=true, field=0)
NNEDI2(dh=false,field=1)
NNEDI3(dh=true).TurnLeft
NNEDI3(dh=true)
NNEDI3(dh=true).TurnRight

########## resize to 16x9 AR, remove sides, add borders
Spline16Resize(2090,940).Crop(24,32,-18,-32,align=true).AddBorders(0,138,0,138)

This does mean producing a new lossless AVI so as everything will probably take at least twice as long as before it could be another couple of weeks before Star Wars is finished. :(