>Somehow, when the root_device is mounted readonly, the changes appear
>to take place, but then mounting the filesystem rw *through any means*
>will result in the old state being restored.


Well, maybe I'm just blind, but ....
you seem to expect that you can write to raw device and the data
you write that way won't be overwritten by contents of buffer cache.
That assumption is false, AFAIK.

If you do  tunefs, you should reboon -n, so that the file system cache is
NOT flushed to the disk (this would actually undo your changes). You
might get dirty root filesystem on the first boot after,
but after fsck you are where you wanted to be.

What I think is happening to you is that on mount update, the superblock
is not read again, so the filesystem runs with the same tunefs options
as it did when it was mounted ro. Then on umount, it flushes
it's idea of superblock to disk, actually overwriting your changes.

"Use reboot -n and not plain reboot in this case."

Jaromir