Hi,
The LIME is certainly a lot of fun, I'm working on a project that needs backlight control for 4.3" LCD and now have a solution that works reliably. The result is very good with no flickering at low levels ( which is a problem with a cheap tablet I use).
(http://pharscape.org/techblog/wp-content/uploads/2014/08/backlight-decay-10-percent.jpg)
You need the pwm-sunxi driver to be available and possibly you also have to modify the LCD driver to disable its control of the backlight - the driver isn't doing anything with it as far as I can see.
For more details and some analysis of the backlight circuit have a look here:
http://pharscape.org/techblog/olimex-4-3-lcd-a13-lcd43ts-pwm-backlight-control/
Cheers,
Paul