Ducking, along with compression, is used quite often for mixing ambient audience mics in live settings like concert/performance etc. We would multi-track everything and always have aud mics onstage and often at FOH. The onstage mics were often ducked during mixing. Side-chained off of tracks sent to a mix bus, or just a 2 track stereo SBD feed if that is all there was. When signal is going through that bus, the ducker lowers or turns off the aud mic tracks. When signal in that bus lessens or stops, then the ducker brings up the volume of the aud mics. You can control the threshold, attack, decay etc on a ducker. Basically reverse compression.
Once you get your settings (particularly attack) it works pretty well just letting it go. You would have to change these for each show though. When reviewing the mix, if you can hear it pump you can always go back and easily fix it. An example of pumping would be something like the commentators in a college basketball game. When the crowd gets really loud you can hear that pump when the commentators break in and the crowd noise abruptly lowers, as they will have the attack and decay times really fast. In a recording situation you can make those more gradual, particularly decay, so it sounds more natural.
One thing to add is that in our situation, we were not looking for adding ambience from those aud facing mics during the performance. They were just there to capture reaction and ambience between songs.