Doesn't help I guess but our 2002 43DS had another unit in the aft garage and the ducting was from there into the rear cabin(s). I feel you have few options. Through aft head or through back of pan cupboard (which would be at least the correct side). Not a lot of help I know. I am just removing mine as the units were rusted heaps and now I have hoses that are just taking up space.
We have the 3 cabin setup (1 forward with desk, salon and master aft) and a single 16BTU air-con under the salon seating, with a dual exit (salon and fwd cabin) and another to the aft cabin.
in a very sunny/hot day it will 'just' cope with the heat (33 C & 95% humidity is the summer in Hong Kong...), but it does work great once the sun starts going down, and for sleeping we never keep it on the coolest setting
the conduit to the aft cabin runs behind the fridge
We asked our techie about this last year and he said it was possible but with new holding tank, new battery and charger/inverter set up installed, new through hulls and a dozen other minor jobs undertaken it has had to go onto the back burner until next year.
I don't want to lose any aft storage space so another unit is not going to happen.
At the moment we simply leave the cabin doors open and set the fans to direct the cool air aft.
We have the same sort of setup with the large unit (16k BTU Webasto) under the settee in the main cabin feeding outlets outboard of the mast, and up in the fwd cabin. We also have a smaller unit (7k BTU) located under the floor inboard of the stove (cooker), with one outlet into the aft cabin. The duct into the aft cabin comes out on the aft side of the stbd hanging locker. Both work pretty well. Both units work off of the same SW pump with separate discharge fittings. I have a 6kw Northern light genset to run them at anchor.