I’ve pointed the problem: maya-render-settings-mental-ray UI create this value by default:
“environment lighting mode” “false”.
The problem is that “false” was not in the official documentation:
http://docs.autodesk.com/MENTALRAY/2014/ENU/mental-ray-help/files/manual/options.html#ibl
“environment lighting mode” “off”|“automatic”|“approximate”|“light”
I can create a special case to deal with that but I would like to know MR community UI guys opinion on this before do anything. 
I think “false” is a “valid value” for mental ray (it understand it as “off”) but IMHO, we should follow the documentation specs and use “off”.
For the others, add this two line (at 1078):
# add elements
for value in stringOption.valueList :
cmds.menuItem( label=value )
+ if stringOption.name == "environment lighting mode" and stringOption.value == "false" :
+ stringOption.value = "off"
# select element
cmds.optionMenuGrp( curOptionMenuGrp, edit=True, value=str(stringOption.value) )
This seems to work with me. :buttrock: