According to this
http://forums.linuxmint.com/viewtopic.php?f=49&t=88350
the sony-laptop kernel module can be given options to achieve your backlight result.
You do this by creating a sony-laptop.conf file in /etc/modprobe.d/
From '/sbin/modinfo sony-laptop'
Code:
parm: kbd_backlight:set this to 0 to disable keyboard backlight, 1 to enable it (default: 0) (int)
parm: kbd_backlight_timeout:set this to 0 to set the default 10 seconds timeout, 1 for 30 seconds, 2 for 60 seconds and 3 to disable timeout (default: 0) (int)