The "idleness", detected by avast!, is "not typing on the keyboard and not moving the mouse". It doesn't matter what the CPU is actually doing - so thing like CPUidle shouldn't have any influence.
As for turning down the monitor... it may be quite a problem. If the (avast!) screensaver is set, Windows simply call it to activate when the preset time has elapsed. However, if only switching off the monitor is selected, I am afraid Windows "won't tell anyone" about it - just switch the monitor off, without calling any module (I may be wrong on this one, however).