We don't directly use that field for anything specific, but i know fmriprep does. so perhaps chris.m can address implications for both fmriprep and nibabel.

But if this is mostly for the file to tell future software: please don't hold me to anything in particular? almost like an unknown flag? then why can't we use NIFTI_XFORM_UNKNOWN

It seems like there is something specific being said, and yet not very specific.