If you are looking for a custom dress, where you want to own the pattern and thus prevent anyone else from copying it, it is only going to work if you don't let anyone take a photo of it. See fashion is not able to be patented or copyrighted. But you can definitely delay how long before someone trys to copy it by working with a custom dress maker and not getting it photographed by the media before your big day. For example, check out http://www.laurengabrielson.com she is nyc. But there are some other custom dress makers out west as well.
Good luck with this project.