Well, I think that it is not a good idea, because when the plaster is dry off, it is very fragil, and if you paint it....can be easily damged or scratched.
Also, if you add the plaster in a wood base, the humity can curve the wood.
Also...is not easy to controle.
I recomend you putty as DAS PRONTO, a clean stuff that can be used with your hands, is light...hard, but with the same look as plaster. You can sculp it with accurancy, you can glue over stones, trees....
Try to search it in your hobby shop...is cheap and very popular.
If you apply Das pronto in a wood base....first you can make the volume with POLIFOAM...the general volumes. Then, you can add the das pronto and you will spend less stuff, and the diorama will be lighter.
All the best