you dont need c to porgram shaders in renderman it has a own rathar easy to use shading language called shading language.
BUt yes no advanced stuff without typing im afraid thie even applies to mentalray, but in gereal ist WAY easier to write renderman shaders than it is to worte mr ones.
Custom is what custom does.
Anwyay the shader in rfm actualy have parametrs for occlusion use them.