ferries are already 3-part, goods mail and pax (automatic assembly in depot, no graphics for the added two), but the 3 parts cannot be assembled in the replacement window (can't click on parts with no picture). Thus they cannot be replaced automatically.
Hmm, I see. An unfortunate state of affairs, and a difficult one to deal with. I shall have to check whether there is any way of having an icon in the new system of hulls and holds, which will probably be the best solution.
Edit: Actually, was being silly: the new system does (and has to) have clickable holds, or else it would not work at all, even in Standard; so, this should be solved with the next pakset release. Apologies that you are having trouble in the interim...