Congrats on your upcoming nuptials!
I think the biggest "honeymoon" surprises on
DCL are for couples having their wedding on DCL (and spending big $$$ to do so). I'm sure you'll get some pixie dust as long as it's noted, and it can make the cruise feel special -- you can always request buttons at Guest Services if they aren't offered.
Please don't take this wrong...I don't want you to set yourself up for disappointment. Sometimes there are some bits of pixie dust for those celebrating a special occasion. But I don't know what info you received about needing 3 weeks notice as there really isn't anything for DCL to "prepare for" in that regard. Yes, you can have the reservation noted as celebrating your honeymoon -- you do that by calling the reservations line and giving them your reservation number. Or you can stop by Guest Services onboard and have it noted. But I don't want you to get all set-up for disappointment. There are lots of people celebrating on the ship each cruise -- weddings, honeymoons, anniversaries, birthdays, etc. If there is something you really want to make it special, book it, order it, plan to make it happen.
Enjoy your honeymoon cruise!