I think a low pressure sender and gauge or warning light would be the best solution as you would instantly see any oil loss in the system, you could also fit a temp gauge for peace of mind.
Just for the record I used the same cooler that you are using
I fitted it to a td5 Defender years ago and it has worked well the transfer usually runs hotter that the R380
I like the idea of a float switch as the absence of oil will be quick and final
I have a spare coolant alarm. The type that just sensors fluid to complete a circuit. I am thinking of using this in a blank plug. This way should there be a leak the level of oil will drop below the sensor and a buzzer and led will sound. Does any one have any comments if this is a bad idea?