Ni sa bula!

IMO it all depends on how and where in Fiji one will get married - it's hard to imagine a wedding dress with train in Fiji. But yes, take it on board, and ask for help on the FJ 747-400 as well. The FAs will probably place it in the forward closet for the trip (big red "C" forward closet on the SeatGuru page), as you will be in Tabua Class (the pointy part of the aircraft, basically wide seats that do not lie flat but do recline sufficiently for some side sleepers, such as myself, kind of like the old AA F "woolly mammoth" seats.; these are older nice -400s leased in from Singapore Air.)

Hmm. Dry cleaners in Fiji. Possibly in Nadi, more likely in Suva. I'd be cautious here. And in Fiji, they don't say "mañana" - they say seqa na leqa, pronounced SENN-guh nah LENN-guh, meaning pretty much the same thing.

The day of departure, check in early at the airport, go to Nadi (pronounce NAHN-dee) to Chef's Restaurant (NOT Saffron, Seafront on Denarau Island or Corner Café, which are more casual dining). Call and book in advance and tell them it is your honeymoon. Trust me - you will be VERY well served, and probably have "Isa Lei" (the Fijian farewell song, and it will have you crying,) as you depart the restaurant.

After dinner, head for the airport and relax in the Tabua Club lounge prior to your flight. (If you are staying at a resort in Pacific Harbour or Denarau, arrange with the hotel to ship the dress back by DHL or the like.)