The test of a microcontroller in plaster is going pretty well, so far. I let the plaster dry out for two days, to get it extra dry, then turned the controller back on. (Note to self: next time mark the wires so you don't have to guess which is which.) Using the Red Bear app, I was able to connect to the BLE module and the RSSI readings change and the phone moves nearer and farther away. I ran it for about four hours and it still worked.
The next step here is to see how hot the Bean gets when connected to the phone to get an idea of how much heat I will be dealing with. Possibilities going forward include: plain plaster strait on the board, heat sink, then plain plaster, and creating a box for the hardware before covering it in plaster. This latter is least appealing to me because I think it will rattle but is probably safest for the controller. The heat testing will give me a good idea if I should forge ahead.