to be honest flech, i have removed dried plaster, simply by vacuuming , more vacuuming a good stiff brush , vaccuming untill no more is visable, then a cold water extraction following all this hard work.
I would suggest leaving to dry completly, "I wouldnt like to think I have wet plaster going through my machine"
somebody may come up with a better suggestion , !!!! but i found this works well
Geoff