Modifying what spawns in a POI dependent on whether it is night or not

[Scyth]

Is there a time of day flag for POIs that I am missing?

 

In spawning.xml for the biomes, I can set different zombies to spawn during the day versus at night. I want to do the same for the various POI, which looks like it is controlled by the gamestages.xml now; but if I'm reading it correctly, there's no ready way to change what zombies will spawn at night.