Have a check in the startup script of the 'ntfs-3g.tcz' extensions which forces a re-run of 'rebuildfstab' if any 'ntfs' file systems are already in '/etc/fstab'.
How about the other way round: ntfs-3g will call rebuildfstab on load via /usr/local/tce.installed/ntfs-3g ? Easier to implement and the call will be done at the right time.
I might have had a slow start today but isn't '/usr/local/tce.installed/ntfs-3g' what I'm referring to as the "startup script of the 'ntfs-3g.tcz' extension"?
With that I meant something like
grep -q ntfs '/etc/fstab' && rebuildfstab in '/usr/local/tce.installed/ntfs-3g', plus something along the lines of
[ "$FSTYPE" = "ntfs" -a -n "$( which ntfs-3g )" ] && FSTYPE="ntfs-3g"
OPTIONS="noauto,users,exec"
case "$FSTYPE" in
ntfs-3g) OPTIONS="$OPTIONS,rw,umask=000" ;;
ntfs) OPTIONS="$OPTIONS,ro,umask=000" ;;
instead of
OPTIONS="noauto,users,exec"
case "$FSTYPE" in
ntfs) OPTIONS="$OPTIONS,ro,umask=000" ;;
in '/usr/bin/rebuildfstab'
(WARNING: code has not been really tested)