Problem writing to ACPI non-volatile memory with /dev/mem - Bad address

[Colin Law]

I am trying to implement the workaround for bug #806032 [1] (Fn +
F[89] does not work for controlling brightness on Clevo laptops) which
uses a script that writes to the ACPI non-volatile storage using
/dev/mem.  This works on Oneiric 64 bit kernel 3.0.0-12-generic, but
not on the 32 bit version of that kernel, nor on the Precise or
Quantal 32 bit kernels (I don't know yet about the 64 bit version,
downloading the live CD overnight tonight).  The command I am running
is
sudo dd if=value_file of=/dev/mem bs=1 seek=3660685688 count=4
the error is
dd: writing `/dev/mem': Bad address

3660685688 is 0xDA31A178 and
sudo cat /proc/iomem
shows
...
da216000-da32bfff : ACPI Non-volatile Storage
...

As I said, this works ok on the 64 bit kernel (from Oneiric at least).
Any ideas anyone?

Colin

[1] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/806032