Here is a script that I wrote at Mikros Images for two projects rendered on Mental Ray. This script is meant to help you manage and create Mental Ray User Framebuffers using the way Autodesk started implement them in Maya 2008 :

With this script you can do several things like add Framebuffers , edit/mass edit Framebuffers, reorganise Framebuffers order, Export/Import your Framebuffers presets (Pretty convenient when you need to keep the same settings from shot to shot).

Source the MelScript and call the main procedure with :

KSL_FrameBuffersManager();

An Online Help is available here : KSL_FrameBuffersManager Help File and a Related Download Page on Highend3D : Highend3D Page