Yes. I would not try to hang something moving/vibrating and quite heavy.

How far is the actual ceiling behind the plaster ceiling. Can get someone to install bracket there. Most plaster ceiling is very thin type hold by metal frame. I put butterfly hook to the metal frame for my lamp and the LED light is very light.

Instead of just fan, now you are adding a wood block to put more weight though it may distribute load. However, you also worries about vibration. More likely this will amplify sound from vibrating plaster ceiling

Add a steel U bracket to cement above to equal the surface of the plaster ceiling and hook the fan to it like one other suggest is a better choice. Cutout of plaster ceiling when working on bracket can be easily patch back.

I though you were talking about putting wood block on plaster ceiling frame. This is more like actually use the real ceiling to hold the load. This should be okay then.

This is when you have access from top like from roof above. If you have plaster ceiling under concrete floor like on ground floor, then you have to make a cut to plaster ceiling to install bracket and then patch the ceiling again.