But it can't be transparent, at least as long as the display device has square pixels. Either it's going to have to be interpolated and stretched by some resampler now or on playback, if you want the displayed aspect ratio correct.
Isn't the preview you're looking at just with some bilinear upsize or something like that?
Just a quick check of a couple things:
bilinear, convert to YUV444
nnedi3 doubled three times to 2048x1792, then Catmull-Rom (bicubic b=0, c=1/2) down to target and convert to YUV444