ehm – the timefactor got a dynamic delay setting called "ducked delay"! it's exactly what you're looking for!
what i am looking for though is that there's a possibility to "invert" the ducking – so that if there's a loud guitar signal, it will be processed with delay, silent signals aren't processed!
but try the ducked delay – it's well done!
no need for the nova delay, which has a dynamic setting aswell, but you can't dial that in very good…