I was able to pack two long gowns for our honeymoon cruise and still do carryon. I also took at least 3 pairs of shoes and clothes for every day.
For a very long time the only luggage we had was his soft-sided carryon-sized bags, and that's what we brought on our cruise, and they still fit quite a lot. For *most* airlines, it's amazing how big a proper carry-on-sized bag actually is! (the cheapest European airlines, and Qantas are the only ones that have had smaller carry-on measurements that we've found so far)
It's not something I do anymore for Disney or a cruise, but I don't think you have to get rid of formal night possibilities just b/c you're doing carry-on-only. Just gotta get the right carry-ons! His Classic Back Door Bag is perfect for such a thing.
And if your stuff is wrinkly, use the pressing services onboard!
Sidenote: in his new cruise book, does he actually suggest doing carry-on only even for a cruise? One of the original points of carry-on-only is so that you're totally portable for the sort of touring from one place to another that he does. With a cruise that's not needed.
We did it because we were taking public transportation from home to the Seattle train station, taking the train to Vancouver, and a cab to the hotel and on to the cruise. (and then reverse) We had to be portable since we would truly be carrying things (this was before roller bags were so popular).