Hi Tricia,
I too booked my trip online so do not have anything re definitive than the research I have been doing here on the boards and other forums.
As far as I can tell though, the only sailings which include Halloween activities are those that are sailing over the 31st. I find no reports from any other sailings. I sailed the Wonder for Halloween of 2001 and there was a costume party and trick-or-treating for the kids.
I think that now that the ships have the Pirates in the Caribbean night, where MANY people dress up for every cruise that the likelihood of additional Halloween festivities on our cruise is very slim. I could be wrong but planning for pirate night is a sure bet and if you have other costumes the kids want to wear, then really give MNSSHP consideration for before or after the cruise.
I will keep my ears to the ground though and share anything I happen to hear.
UPDATE: Just came across this from 2009 about activities for the whole of October on the Magic and Wonder
http://disneyparks.disney.go.com/blog/2009/10/disney-halloween-at-sea/ still looking for anything about the Dream
Christopher