Workflow tip: how to get a quick'n'dirty preview in the CR2 before the (time-and-space-consuming) cr2hdr step?
Method 1:
exiftool IMG_6706.CR2 -PreviewImage -b | convert - -resize 100%x25% -resize 100%x400% preview.jpg
exiftool IMG_6706.CR2 '-PreviewImage<=preview.jpg' # -overwrite_original
rm preview.jpg
Question for exiftool and/or bash experts: can this be done in one command, without a temporary file?
How it works: resizing on the Y axis gets rid of the stripes; resizing it back gives a nicer preview (could be useful when selecting a few "keepers" from a large number of files).
Run time: about 4 seconds, single thread on Core i7-7700HQ.
Method 2: use cr2hdr --fast (or --fast --fullres)
cr2hdr IMG_6706.CR2 --fast # or --fast --fullres
dcraw -b 16 -c IMG_6706.DNG | convert - preview.jpg
rm IMG_6706.DNG
exiftool IMG_6706.CR2 '-PreviewImage<=preview.jpg' # -overwrite_original
rm preview.jpg
Run time: about 9 seconds on the same system (or 20 seconds with full processing).
Of course, the above can be wrapped into scripts to process all files in one directory, tweaked to run in parallel and so on.
Pixel peeping:
- original preview from the CR2:

- with method 1, using the following resize factors:
a) -resize 100%x50% -resize 100%x200%
b) -resize 100%x33x33% -resize 100%x300%
c) -resize 100%x25% -resize 100%x400%

- with method 2, using the following options for cr2hdr
a) --fast
b) --fast --fullres
c) default (full processing)

For my own use, I've ended up with this:
#!/bin/bash
# Dual ISO preview
# To process all files from a directory, with GNU parallel:
#
# ls *.CR2 | parallel dual_iso_preview.sh
# exit on error
set -e
# base file name, without extension
fn=$(basename $1 .CR2)
# backup old previews
dcraw -e $1 && mkdir -p original_previews && mv $fn.thumb.jpg original_previews/
# quick processing, just good enough for a half-res preview
cr2hdr $1 --fast --fullres
#dcraw -w -b 16 -c $fn.DNG | convert - $fn.jpg
ufraw-batch $fn.DNG --shrink=2 --exposure=auto --wb=camera --out-type=jpg --compression=70
# replace the preview with our own (caveat: it will delete the old preview!)
exiftool $fn.CR2 "-PreviewImage<=$fn.jpg" -overwrite_original
# cleanup
rm $fn.jpg
rm $fn.DNG
Make sure you've got backups before running this! (just in case)