General consensus appears to be that the Commando doesn't really need one as it 'copes' with the heat very well.
Also, provided one uses good, modern (IMHO synthetic) oil in a Commando, the oil will never be damaged be the heat created. So the oil itself does not need cooling. In fact, many engines may actually over cool the oil.
Having said all of that, I personally believe there is a good case for cooling the oil being fed to the cylinder head as this will help to lower cylinder head temperatures which will give and extra safety margin for modified and / hard pushed engines. If I look at my own bike, the head is basically the same as designed for a low compression 20+ BHP 500cc yet it now has to cope with 10.5:1 CR 60BHP and 850cc... so I think it could use some help...!
Pipe routing for this would be simple, simply extend the existing rocker feed directly to a cooler mounted under / close to the coil location, then out of the cooler to the rocker feeds.
This would only cool the oil going to the head. The majority of the oil would return to the tank un cooled, thereby negating the over cooling worry and need for a thermostat, but it would, I believe, give benefits to lowering the engine temperature.
I have done this before on Nourish engined race bikes with great effect. I have moved my coils on the Commando near to the battery to make way for a nice neat cooler... that is currently sat on the shelf waiting to be fitted... !