InteractivePhotorealisticRender (IPR) is not the same as "render region" as far as I know.
So we are thinking of two different things, as the newly released mental ray does not support the IPR feature.
"Render Region/Resolution Settings - mental ray renders now respect the Render Region or Test Resolution settings in Maya's Render View."
Quoting from mental ray manual, about things that was fixed.
I do not support Bush! Nor Saddam! Long live UN!