Now, what exactly do you mean by stencil like :)
If you mean semi-transparent - yes, Q3A has windows in the base game you can use as start point.
If you mean something more like cel shading (cartoon), yes, you can do that as well. For a few examples, see:
An ever decreasing dome or cylinder is not something that already exists (as far as I know) in Q3A. That is something you will have to write yourself. What ever you come up with will also need to map the shader to that surface.
I would see if you can get a hold of the PainKeep Arena source code or check the Kamikaze code in Team Arena. Both might be a good starting point for what you want to do, just on a match larger scale.