I can see this being mostly bad viewing for wheelchair users. I would like a good answer to this, as well. I haven't seen the show, but I almost think being on the porch of the Cyrstal Palace might be the best option. You could get right at the railing and even if someone was down below it wouldn't block your view; because, you are higher up. Another problem will be the porch railings, but maybe if you look up you would get an ok view out of it.
Another thought would be going to the dessert party. Again, you are above the crowds, but you are definitely farther away then Cyrstal Palace. Not sure when they allow people in for the dessert party. Also, they may rope that area off ahead of time and you may not be able to get in there except for the fireworks time.
Finally, if you don't care about the fireworks viewing as much then being in front of the stage area of the castle might be your best bet. You'll have to strain some, but you should be able to see a lot of it and you would be up close.
It would be nice to hear from others actual experiences on this one.