Re: Plaster sculptures outside?
The 1893 World's Columbian Exposition in Chicago, and probably a number of other such World's Fairs during that period, had architecture, architectural ornamentation, and sculpture made of "staff", which was plaster over lathe, cornhusks, and/or burlap. They lasted for the year or two or more for which they were intended.
In fact, the Fine Arts building from 1893 was converted using permanent materials into the Museum of Science and Industry. So, although in disrepair after 40 years, it hung on pretty long for plaster in a 4 season climate.
So, prepared well, as a temporary medium it should be everything but evaldart-proof.