This was asked on the rockbox forum and the backlight hardware is different for the H100 and H300s =\
If i understood correctly the H300s lcd is connected directly to the power management chip and you only get the 2 -> 15 steps. The H100s can control thier PWM chip better than the 300s.
I guess you could have it fade to 2 but if you go into the backlight brightness settings and hold left and right you'll notice its not a smooth fade.
The guy who told me this said he would try and work on a backlight->low backlight->off effect like some cellphones have. Which would be just as good, maybe even better.
Edit: heres the link
http://forums.rockbox.org/index.php?topic=2443.0