I have read that the old plaster craftsman used plaster for exterior architecture details. They would soak them in linseed oil. I have some plaster column capitals in my yard. I sealed them with an epoxy paint and after 8 years they are still there (with some issues, I will replace them in a few years). If it's only for a few months and you use a very hard plaster and you seal it well...you should be fine.
Here are a few links to get you started