Very, very interesting that Amel built most of the saloon and forward cabin pass through/ducting even on those boats that neither have forced air nor heating from the factory! Since the hard part on any unheated sailboat is obviously finding the space for these thick air ducts, I guess Amel already did well over half of the job compared to many other sailboat makes.
This means that the decision to go Hydronic for non-heated/non-force-air-ventilated boats is not as self-evident as I had thought; those wanting to install an Airtronic would not be starting from zero at all. But people wanting to install Hydronic could also try and profit from the existing Amel buildout, so it's a benefit for them as well.
Now, as to the ducting to the aft cabin: SM's with the heating or the forced-air option have a nifty, thick stainless steel tube that runs fore to aft, across the aft head, above the toilet seat. This is the duct that connects the furnace in the cockpit locker to its exhaust in the aft cabin, immediately above the A/C exhaust. When the Airtronic gets turned on, this tube gets warm, providing a bit of warmth to the area. At some point after hull #350 was built, Amel installed a subsidiary sliding door/grilled exhaust longitudinally on the tube so that some warm air can issue inside the aft head, for even better heating, if so desired.
SM2K N°. 350 (2002)
Under sail, Sicily to Valletta