OK, just so the few fellow 70D users who experience the issue can find the fix here as well:
Thanks to Danne's
6+ years old post I found the workaround for my LR DualISO problem.
If not using this plugin:
1) Convert your DualISO CR2 files with the latest cr2hdr.exe WITHOUT using the --compress command
2) Run exiftool -uniqueCameraModel="Canon EOS 6D" -ext dng . (so that you change the relevant EXIF tag for the resulting DNG files as per Danne's instructions)
3) Import your files into LR the regular way
4) Optionally apply lossless DNG compression in LR via the Update DNG Preview & Metadata command
This way you get properly imported DualISO DNGs (who will still register as shot on 70D, BTW). Compressing them before importing into LR somehow breaks this 'fix' even if you do change the tag.
If using this plugin:
1) Export your DualISO CR2 files as usual
2) Run exiftool -uniqueCameraModel="Canon EOS 6D" -ext dng . (so that you change the relevant EXIF tag for the resulting DNG files as per Danne's instructions)
3) Refresh your LR files with the now externally changed metadata and regenerate the previews
4) Optionally apply lossless DNG compression via the Update DNG Preview & Metadata command