I know that nobody has successfully created a reaction that applies liquid poisons to ammunition or weapons before. I have a theoretical solution, but I'd like some input before I try implementing it since I'm not very familiar with how interactions work.
We know that liquids can get applied to items if the items are on the same tile as a barrel containing the liquid, and the barrel is then removed. I first tried creating a custom reaction that called for bolts as a preserved reagent and a liquid as a regular reagent, and that produced liquid (not in a barrel) as a product. This was not successful; the liquid was produced correctly in the workshop, but when the bolts were removed they were not covered with liquid and the liquid got retained until the workshop was deconstructed.
So here's my thinking:
Add a reaction that uses the same reagents, but that instead produces an instant-boiling liquid that conveys an inhaled syndrome.
The syndrome has a very brief period in which it conveys a CE_CAN_DO_INTERACTION.
The interaction forces an immediate material emission of the same type as the liquid used in the reaction.
The dwarf will immediately emit the liquid on the tile that they're standing on, which should coat any items in the workshop and on the tile (and incidentally also coat the dwarf).
This type of syndrome could also be used on food and beverages to coat dwarves with a substance when they eat or drink something, or be added to a creature attack to cause a foe to become coated in something nasty as a result of getting bitten.
Am I overlooking something here?
Edit: another thought I had was to exploit tags on venom-containing vermin by giving them an organ that is composed of the desired substance and has [TISSUE_MAT_STATE:GAS], then create a special bolt-requiring reaction for them that includes that organ as a product, which would (in theory) immediately condense into venom in an inversion of what happens with cave floaters when they are butchered.