I need to eat my words. The EXPTIME issue I mentioned in Siril is not at all related to Astro-TIFF. It is a recent change where the total stack time is now saved in LIVETIME and the total count of images is in STACKCNT. So for a normal stack, STACKCNT * EXPTIME = LIVETIME. Those fields are in the FITS header regardless of whether Astro-TIFF or FITS source files were used.
My wish to convert from FITS to Astro-TIFF via a command line tool still stands.