Cycles: Is there a way to find the angle between incidence and reflection rays? Is there any point, be it through cycles nodes, OSL, or even somewhere in the code of Cycles itself, where the angle is known between the camera/incidence ray and the reflection ray for a given bounce?

Note that I'm not concerned with the surface tangent or angle of the ray to the surface, unless those are necessary to the angle between rays. 

If this data is not exposed to the user but is known in-engine then I would be grateful for where in code that would be.
The angle between incidence and reflection ray depends on the surface:


If the surface is glossy with roughness 0 (perfect mirror), this angle is double the angle between incidence and surface normal:


If the surface is anything else, the reflected rays scatter based on probability - based on BSDF function (rough surface, diffuse surface,..). You would have to look inside Cycles code to trace these rays individually. The best help you will get directly from devs on IRC channels or through mail-list, look here who owns the code: Module owners.