You mean something like this?
http://www.summitracing.com/parts/ti...cIwhoCAaLw_wcB
It might be a good idea anyway as the MGR is in the rear and not getting much actual airflow for cooling...
Looking at the case, it seems that an external oil outlet could be placed near the bottom of the ring gear, feeding back in at the end of the motor shaft, does that seem right?
Are you really machining a custom case just for your own needs?