sorry for your injury! it's been interesting reading this thread though b/c I always assumed Disney wouldn't let people on rides w/casts (we went to a non Disney amusement park when dd was younger, and when she tried to get on rides she was refused by the operators-when I asked to speak to a manager of the park they got one who explained their insurance prohibited it).
that said-if you do get the cast off or get a smaller version prior to your trip: keep in mind that the skin that's been inside the cast will not have been exposed to the elements for several weeks so make sure to apply extra sunscreen to the newly exposed area to prevent a nasty burn, some good deep penetrating lotion will help w/any dry skin issues and we also used something akin to desitin to deal w/reactions to pool chemicals and such (only seemed to affect her for the first week or so until the skin got reacclimated to being exposed everything). if you get completely out of the cast check w/your doctor about some type of protective apparatus you could wear while in the park or doing other activities-when dd got her cast off the orthopedic center she went to made her (same day) a little custom molded support for the area of her arm that was basically made of hard plastic on the outside w/soft foam on the inside, it was hinged on one side w/Velcro straps on the other. for the most part she used it when she was going to be in situations/doing activities where she might get bumped or jostled on the affected area.
the paranoid over planner in me also says it might be a good idea for you to review your health insurance to make sure you're covered outside your home area (in our case 'service area') b/c some policies don't cover non emergent services outside your area (and I know my p.i.t.a. insurance would say that if I had a pre-existing break and opted to go to an amusement park where I reinjured it all the out of region care would be on my dime). if you've got
travel insurance don't rely on it-most have a provision that disallows anything you've been diagnosed with or received treatment for w/in 60-180 days prior to leaving on the trip.
hope all works out for you-have a great time but take care, I'm still healing from "minor" orthopedic surgery 5 months ago-you don't want to have to go that route.