Looking around, chntpw seems like it should do the job, even
though it was originally meant just for changing passwords (good
old Microsoft security).
http://www.cgsecurity.org/wiki/Chntpw_for_Dos
A better description of it is on the site where I found it
(along with lots of other fun DOS FS tools):
http://reimagery.com/fsfd/disk1.htm#filesys
Though chntpw is based on a very old Linux NTFS driver anyway,
so if making a bootdisk I'd recommend Linux where appropriate. I
don't deny that DOS can be better some times though...
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