Stefano Jannuzzo has created and shared a Ray Trace Shooter Shader for Maya and Softimage, called sj_trace. SJ_trace will allow you to shoot any kind of ray along any direction, the shader is documented for Softimage currently, however it is available for Maya as well.
“sj_trace is a shader letting you trace several types of ray in several directions. In the standard tracing shaders, the tracing direction (for instance for reflections or refractions) is computed internally by the shader. Also, the starting point of the ray is alway the current hit point. sj_trace instead exposes the type, origin and direction of the ray to be traced.”
