Well, what immediately comes to my mind are using Blobby particles with a relatively low radius, and a relatively low surface threshold so it appears more like saliva, and not one big mass of goo.
You may have to use more than one surface emitter, but use the mouth pieces as GOALS.
Then, simply tweak the goal weight to your desired setting so that it appears to drip saliva, but not stick all the way to it, and some bits fly off due to animated velocity driven by the character itself.
I would also ask Mike McKinley about this, as he works for a company that may have done something similar in the past that he may have another technique to try as well, good luck!
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS