On some thinkpads, the kernel supports altering brightness of the laptop monitor via acpi. I am not sure if yours is included (if not you would have to consult the manufacturer) but try this
1. ensure that the CONFIG_ACPI_VIDEO and CONFIG_VIDEO_OUTPUT_CONTROL kernel config parms are both set in your running kernel
2. Now (after rebooting if necessary) do this
find /proc/acpi -name brightness -ls
If that found a file (or files), then for each file, cat the file to see what it contains. If you are lucky it may contain either some instructions on how to alter the brightness of a single number.
If the latter, then try
echo number >file
where the number you put is a little (not too far) lower or higher than what was currently there, and see if you notice a change.
If this works, this is a (awkward) way of doing it and you could then experiment with binding an acpi event to some keypress (hotkey) to perform the echo command as an action.