I posted this !!SCIENCE!! discovery in the Dwarf Mode Discussion forums, but it's pretty modding related so I'll duplicate it here.
You can have your reactions/interactions/anything else reference the generated materials raws, if you know what they will be called. For example, evil rains are all called EVIL_RAIN_#, forgotten beasts get a similar naming scheme, and so on.
So, if you like the random rains/fogs, but want to adjust how the reanimation works in evil biomes, you can set worldgen to create 0 regional interactions, then create your own regional interactions, but still have said interactions refer to the randomly generated evil weather. For example, if you set generated regional interactions to 0 (but leave the fog and rain counts as not-zero), and then have this in an interaction_.txt file
[OBJECT:INTERACTION]
[INTERACTION:EXAMPLE EVIL RAIN REGION]
[I_SOURCE:REGION]
[IS_REGION:EVIL_ONLY]
[IS_REGION:SAVAGE_ALLOWED]
[IS_REGION:ANY_TERRAIN]
[IS_FREQUENCY:100]
[I_TARGET:C:MATERIAL]
[IT_MATERIAL:MATERIAL:INORGANIC:EVIL_RAIN_1:WEATHER_FALLING_MATERIAL]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:C]
[IE_INTERMITTENT:WEEKLY]
Then if that's the only regional interaction, all evil regions in your world will use whatever EVIL_RAIN_1 is as their falling material interaction, and it will be different for each world. Now the error log will constantly complain about your raws having invalid materials, but it works all the same.
Why do this? Well, it lets you customize the animation regional interactions without losing the randomness of the generated weather effects making each world unpredictable
Of course a better way is probably to write your own script to generate random weather materials/syndromes, but this is WAY faster