I plan to finally dive into mental ray and get a little wiser.
Please contribute with your godlike knowledge of mentalray
FIRST SOME ISSUES:
mentalray Beta doesnt seem to support the render-region function in maya. Which means you have to (if I'm not wrong) render whole images, every time you render with mentalray.
SO FAR... (About Final Gathering)
I have found out that you can use a materials "incandesence attribute" to act as a lightsource. BUT You need to create a light, else the default lights appear and wash the scene with light. Set the intensity of the created light to "0". You need not turn on any photon emitting with final gathering alone (it seems)
ABOUT THE RADIUS SETTING...
-I have read at several places that the MAX radius should be like 5-10 % of your scene's size in maya units.
-MIN radius should be like 10% of MAX radius. Im not sure, but it was also said that the min radius should be made smaller to take into account the smaller objects in the scene. But I dont know (at this time) what the differences will be, if MIN radius is too big for your smaller objects.
The pic below shows a final gathering rendering with one material, having some incandesence. Im sure I can figure out how to make the "light source" dissappear, but I not sure how at this point.