Okay, got what I wanted working through some trial and error. Apparently GUID_VIDEO_CURRENT_MONITOR_BRIGHTNESS is not the correct thing to use and for some reason the functions won't work with it (even though it's in my header file under the subgroup). So, I'm not really sure at this point what the purpose of it is, but don't use it for this case.
What does work is the same function but using GUID_DEVICE_POWER_POLICY_VIDEO_BRIGHTNESS. There's also GUID_DEVICE_POWER_POLICY_VIDEO_DIM_BRIGHTNESS if you want to change the brightness of the "dimmed" state.
Using this define instead, I get my desired behavior.