You have a long time to sort this out. I recommend finding a box that is within the "regulation size", for the future Mrs MDJ's sake and for your own sake. She would be very, very, very annoyed if she is told she cannot bring it on board. So why risk it? You can even have a box custom made and I think it would be well worth it to reduce the risk of upsetting your future Mrs MDJ.
In fact, since you are the "frequent flyer", if she ends up having to check it in and it makes her worry for the entire flight, or worse, if it goes astray, YOU will be the one to be blamed. Trust me, many women just before a wedding can be quite sensitive, and potentially irritable because of the stress of wishing the day to be absolutely perfect. You will have a no end of telling off, blaming, tears and all sorts if this goes wrong.
So, show some initiatives and offer to have a good box custom made for her dress, citing the reason that it should not risk being put in the hold, and you are sure to score some brownie points, and you do not risk being blamed or at least having a very upset bride.