Take a look at 8019 (food: poison), it has "'Poison' domain_special 3" spell memorized. Essentially it will call Spell352 - Poison.py: target_item.obj.condition_add_with_args( 'Poisoned', poison_index, time_to_secondary, poison_dc ) Which is condition I altered a bit. Now 3 is poison index from T+ new file - https://github.com/GrognardsFromHell/TemplePlus/blob/master/tpdatasrc/tpgamefiles/rules/poisons.json - "Blood Root" poison. Therefore you can easily expand the file. Just remember to place it in the <game>/overrides/rules/poisons.json, if you wish to override files from Temple Plus.
Thanks guys, but I got it going by just adding a flag at san_use and then checking for it. Too easy. @Allyx - individualised poisons on Daggers of Venom is the next task