DH's best mate and his wife got married at their engagement party. The party was at his mum's house and when the time came they asked everybody to go to the front of the house. The bride was wearing a pretty dress for the party but had changed into a wedding gown. They had told some close friends who they needed to perform certain tasks, but neither set of parents had any idea. After the ceremony the also announced their pregnancy. It was a beautiful, relaxed day. Very Australian with the smell of lamb roasting on a spit coming down the driveway lol. I say, if it's what you want, go for it.