As of Plants 2.8.0, the Magical Brewing Cauldron has recieved CraftTweaker support.
This allows pack devs to customize which potions a specific color can give. Each EnumDyeColor now has a weighted list associated with it. This list is what the cauldron uses when generating the potion stack. Potions are pulled randomly with more favor towards those with a higher weight. An example script is below.
import mods.plants.BrewingCauldron;
BrewingCauldron.addWeightedPotion("red", <potion:botania:bloodthirst>, 5);
BrewingCauldron.addWeightedPotion("black", <potion:botania:emptiness>, 90);
BrewingCauldron.addWeightedPotion("red", <potion:minecraft:glowing>, 5);
BrewingCauldron.clearWeightedList("black");
BrewingCauldron.removeWeightedPotion("red", <potion:minecraft:instant_health>);
There are three methods in total. addWeightedPotion(String, IPotion, int), clearWeightedList(String), and removeWeightedPotion(String, IPotion). In each case, the string must represent an EnumDyeColor, it is not case sensitive.
addWeightedPotion will add the provided potion to the specific color list, with the given weight.
clearWeightedList will clear the entire list for that color.
removeWeightedPotion will try to remove only a single potion from a color list. Not exactly useful currently since each list only contains one color by default, and you could just use clearWeightedList to the same effect.
All scripts are run during the FMLInitializationEvent. Clears are processed first, then removals, then finally additions.
The default color -> potion map may be found here. All default entries have a weight of 10.