Would be worth investigating the exit temperature of the airflow as it is a short distance that the air flows over the oil cooler fins.
Since the air gets pulled over the oil cooler fins the engine is an air/oil cooled engine. So, it is effectively a heat exchanger at that point as you know.
Certainly, would be a very safe option and I don't think the decrease in efficiency would be a problem in cold temps at all.
FYI: I thought I saw on a engine diagram somewhere that there are additional oil ports into the engine case. U could also use those additional ports for another safe alternative if there is such a thing as an aftermarket oil heater that you could connect into your block? This additional "heater" would function as an additional oil cooler for the engine and a oil heater for your cab as you could run the flexible oil lines from the engine block to your cab. Out of the box thinking I guess.