I made a system on a server a while ago, and posted for someone who requested to look at it back then.
http://nwvault.ign.c....Detail&id=3778
It uses a number of different poison vials that add OnHit acid splash to the weapon. I then rescripted part of acidsplash to do different things depending on which vial was used.
If nothing else, it's an example of custom poisons.