The hack I came up with (following also the ISO boot thread with ruymbeke's binaries) is to tack onto linuxrc in minirt.gz the following just before the call to exit():

# SMartin - HACK below just forces unmount of ntfs drive
# we are booting from (if we are booting from one)
# and gets mtab back in sync if its messed up.
/sbin/umount.ntfs ${LOOP_SOURCE} > /dev/null 2>&1
cat /proc/mounts > /etc/mtab



Good luck.
I can provide a minirt.gz for you if need be.