As an oversight, I forgot to include a horizontal flat frame when shooting them for a bunch of lenses and apertures (as in holding the camera horizontally when shooting the illuminated target) but do have the matching vertical ones. as RAW files).
Can I use the vertical format flat files when processing horizontal lights?
(or should I set up again and shoot some - or find some way to edit the orientation field in the EXIF data?)
EXIF orientation does not matter for astronomy, what matters is the orientation of the sensor with regards to the optical path of the telescope.
If you rotate the camera, the dust particles and vignetting artefacts will not be at the same position on the sensor; the flat frame will be useless. I’m afraid you’ll have to shoot some new frames.