As far as I remember, the issue is not actually writing to the disk bypassing the FS. It is the fact that XFS can consider files stored only in the journal as committed on the disk, and then later on moves the data from the journal to the final locations. GRUB 1's XFS module used to ignore the journal, so it might not see the most recent versions of all files. I also don't know if the grub installation procedure might embed sector locations for boot files while they still exist only in the journal.
Sadly, I don't know if this problem has been solved.