USING RAY TYPE SHADER TO EXCLUDE OBJECTS FROM RAY CALCULATIONS
This shader can be assigned to surfaces to control reflection, refraction, light, shadow, environment, final gather, photon, custom. For example, to cast FG to a surface but have reflections from another surface...
*On your IBL, turn off visible in reflections
-assign a surface shader to your reflection poly sphere
-set the transparency of the surface shader to 1
-assign to the "ray type" node to the color of the surface shader (you will find it under mental ray/ miscellaneous)
-check "enable reflection" and "enable refraction"
-set eye to black
-assign a maya native envirnemnent map (or a reflection map) to the reflection and refraction slot.
-don't forget to turn off the secondary reflection of the IBL and thank the mental ray guys for offering super mega amazing complexe hypertechnical plugthisandtwistthat long workarounds just to achieve a simple effect.