I can't seem to find any way to use the acSendKeys call, but I added acGetMonitorBrightness and acSetMonitorBrightness in 2.7.7. Although, it works on my monitor, it doesn't seem to work on my laptop screen...so you may need to just use S+ to call NirCmd utilizing the changebrightness function.
My test script:local iCurrent = acGetMonitorBrightness(nil, gsx, gsy, 1) -- 1 = get current value
local iMin = acGetMonitorBrightness(nil, gsx, gsy, 2) -- 2 = get minimum supported value
local iMax = acGetMonitorBrightness(nil, gsx, gsy, 3) -- 2 = get maximum supported value
acMessageBox("Current: "..iCurrent.." | Min: "..iMin.." | Max: "..iMax)
acSetMonitorBrightness(nil, gsx, gsy, 50) --set brightness to 50%