At the start of your /etc/acpi/handler.sh, put a
logger -t acpi -- $*. Then do a
/etc/rc.d/acpi restart. Then press the function keys in question, then check your syslogs to see if anything shows up. acpi doesn't detect all the special keys on my laptop. Some of them are detected as regular keys instead. A few show up both in acpi and as regular keys. If fn-F12 is detected by
xev (as being something different than plain F12), but isn't detected by acpi, you could look into xbindkeys. How to get acpi to detect more than it's already detecting I don't know.
I used an earlier version of Ubuntu, in which they ran a daemon that watched for such keypresses; so basically they were using their own version of xbindkeys.
EDIT: I see that acpi_listen makes a lot of this easier; it's like xev for acpi.