keep in mind that it wont spread as a contaminant. I believe the decoration is kept in stasis like the egg whites. But if you're interested in coating weapons with poison, well, I think I've found a way to do it, but it may not work and if it does there will be a few caveats. The general idea is to make a one tile workshop and make a reaction for it that produces a liquid poison and requires a weapon as a reagent. the weapon is preserved via [PRESERVE_REAGENT]. The liquid is NOT produced to a container. the workshop must then be deconstructed, but when it is the contents will fall to the ground much the same as if you prematurely deconstruct your wagon resulting in the weapon on the ground and a pool of the poison. The issues I'm foreseeing is are that A) the poison MUST be dorf safe, as there's no way to make ONLY the weapon and ONLY the blade coated, and B) it may take some finagling to get the poison on to the weapon, as I'm not sure if it will toss the weapon to a different square or the same square as the poison. I need to put in the workshop and do some testing, but the theory is sound as far as I'm aware. If nothing else it can be used to make a poison lined entrance to your base.
EDIT: preliminary tests were met with mixed success. The reaction did produce pools of the contaminant (in this case, blood), but the weapon did not get a covering even if it landed in one of the pools, though the travel of dwarves did spread the blood, so it should act as a decent delivery system if nothing else. It produces the contaminant in the squares surrounding the workshop as well as the workshop itself, if you produce enough of it before deconstructing. I'm gonna make a separate thread for this; All of dwarfkind deserves to know!