From all i have read above it appears to me that all BMW are saying is increased cooling/air flow is required for the expected work load while towing, more of a issue in warmer climates i would of thought.
If that's the case why not simply fit a extra cooling fan?
I'm not familiar with the BMW cooling fan, i assume its the viscous coupling type as fitted to the Audi i had? If so simply fit a switch to operate the extra fan while towing.
Or if the fan cuts in on coolant/engine temperature, slave the extra fan of that via a relay.
What i think is more of an issue with aftermarket tow bars now is the wiring interface to the 13 pin socket.
Again assuming the BMW has the Can Bus system, if the BMW specific loom is not used, and the ECU is damaged then a couple of thousand pounds will be needed minimum to replace it, out of YOUR pocket.
Unfortunately if you buy into the prestige market, the extras are always at inflated prices, along with the labour charges.
I always pay more attention to the water temperature while towing, any car while towing will be working harder.