In my opinion, dresses that are full length with long sleeves (although kind of hard to come by) are usually 'hijabifyable'. I came across a range of ball gowns from the site http://www.buyweddingdress.net/ and wanted to share some here:
Ok so some of the sleeves aren't full length and some are sheer, but you could sort this out with a top underneath. If the party you're attending is all female, you could simply wear a coat over the dress and discard this and the hijab when you arrive.
My Fave dress? The dark red one!! It's so beautiful.
The site also does a beautiful range of wedding dresses with hijab which I will do a post on in the future Insha'allah.