Why not use the oil cooler with a capacitive sensor array, then calculate total trans volume by difference in level change in oil cooler. This would IMO provide a very sensitive oil quantity leak detection system. Only downside would be when cooler bypass is activated, so I would also use localized thermocouple/RTD elements at strategic drive component to measure potential "hot-spots".
We use these condition-based monitoring techniques with main power generation/reciprocating machinery in the oil and gas industry to provide continuous monitoring and alarm indications as well as safety shutdown systems on high-value equipment.
Max